You are writing a check every month to an ex-spouse. You have heard through mutual friends that they are living with someone new. If your suspicions also involve infidelity, read Infidelity Investigations: What to Expect — the surveillance methods overlap significantly. And before you consider any DIY tracking, read GPS Tracking Laws in Illinois first. You have seen the social media posts. You have a strong feeling that the legal basis for those payments no longer exists.
But a feeling is not evidence. And in Illinois family court, evidence is everything.
This article explains exactly what Illinois law requires to terminate spousal maintenance based on cohabitation — and what a licensed private investigator can do to build a case that holds up.
What Illinois Law Says About Cohabitation and Alimony
Under the Illinois Marriage and Dissolution of Marriage Act (750 ILCS 5/510(c)), spousal maintenance — commonly called alimony — terminates automatically upon the death of either party or the remarriage of the recipient. But there is a third trigger that many people do not fully understand: cohabitation on a resident basis with another person.
The statute reads that maintenance terminates if the recipient "cohabits with another person on a resident, continuing conjugal basis." That phrase carries significant legal weight, and courts have interpreted it carefully over the years.
Here is what it means in practice:
"Resident" means the person is actually living at the location — not just visiting frequently. Courts look for evidence that the new partner has established a primary residence with the recipient, not merely that they spend time there.
"Continuing" means the arrangement is ongoing and stable, not a temporary or casual situation. A few overnight stays do not meet this standard. A pattern of consistent cohabitation over weeks or months does.
"Conjugal" means the relationship is romantic or intimate in nature — not a roommate arrangement or a family member moving in.
All three elements must be present. That is why documentation matters so much. A single photograph or a neighbor's opinion is rarely sufficient. Courts want a pattern of behavior, documented over time, that establishes all three prongs of the standard.
What Happens If You File Without Enough Evidence
Family law attorneys see this mistake regularly: a client comes in convinced their ex is cohabitating, the attorney files a motion to terminate maintenance, and the opposing party simply denies it. Without solid documentation, the motion fails — and the paying spouse is often ordered to cover the other side's attorney fees.
Filing prematurely does not just fail. It can make the situation worse by alerting the recipient to the investigation, giving them time to adjust their behavior, and creating a legal record of an unsuccessful challenge that complicates future attempts.
The right sequence is: build the evidence first, then file.
What Evidence Illinois Courts Actually Accept
Over the years, Illinois courts have considered a range of evidence in cohabitation cases. The strongest cases combine multiple types of documentation:
Surveillance Documentation
Timestamped photographs and video showing the new partner's vehicle consistently present at the recipient's residence — particularly overnight and on consecutive days — is among the most persuasive evidence available. A licensed PI conducts this surveillance legally, documents the chain of custody for all evidence, and produces a written report suitable for use in court proceedings.
License Plate Recognition (LPR) Data
Through IRBsearch's nationwide LPR database, I can query months of historical vehicle location data in minutes. If a vehicle registered to the new partner has been scanned at the recipient's address dozens of times — predominantly overnight — that data establishes a pattern of presence that is difficult to dispute. I have used LPR data in multiple cohabitation cases in the Chicago area, including a Cook County case where 47 vehicle sightings over 90 days helped terminate a $2,200/month alimony obligation.
Address and Utility Records
Public records and investigative databases can reveal whether the new partner has updated their address to match the recipient's residence, whether utilities at the address have been changed to include the partner's name, and whether the partner's vehicle is registered to that address.
Social Media and Digital Evidence
Publicly available social media posts, check-ins, tagged locations, and photographs can corroborate physical surveillance. Courts have accepted social media evidence in cohabitation cases, though it is rarely sufficient on its own. It is most effective when it aligns with and reinforces documented surveillance.
Witness Statements
Neighbors, mutual acquaintances, or others with direct knowledge of the living arrangement can provide supporting statements. These are useful as corroboration but are rarely the primary basis for a successful motion.
The Role of a Licensed Private Investigator
An attorney can file the motion. A judge decides the outcome. But the evidence that makes or breaks the case is gathered before either of those things happens — and that is where a licensed PI comes in.
Here is what I do in a cohabitation investigation:
1. Start with database research. Before any surveillance, I run LPR queries, address history checks, and vehicle registration lookups. This tells me whether a pattern already exists in the data and where to focus a targeted surveillance operation. To see this process in action, read the LPR Case Study: How License Plate Recognition Helped Save a Client Thousands in Alimony.
2. Conduct focused surveillance. Rather than open-ended stakeouts that run up costs without producing results, I use the database intelligence to identify the highest-probability windows for documentation. Two or three targeted sessions, timed correctly, can produce more usable evidence than weeks of unfocused watching.
3. Document everything with legal admissibility in mind. Every photograph is timestamped. Every observation is logged with date, time, and location. The chain of custody for all evidence is maintained from collection through delivery to your attorney.
4. Produce a written investigative report. Your attorney receives a complete package: the LPR data, the surveillance documentation, and a written summary of findings that can be attached to a motion or presented in court.
5. Remain available for follow-up. If the case proceeds to a hearing, I can provide a sworn affidavit or, if necessary, testify regarding my findings and methodology.
How Long Does a Cohabitation Investigation Take?
It depends on the case. If LPR data already shows a clear pattern, a targeted two-day surveillance operation may be sufficient to corroborate it. If the subject is cautious or the pattern is less established, additional time may be needed.
In my experience, most cohabitation investigations in the Chicago area reach a documentable conclusion within two to four weeks of engagement. The LPR database research is immediate — the surveillance is what takes time, and I keep it as focused and efficient as possible.
What This Costs Compared to What You Are Paying
The most common question I get from clients in this situation is whether the investigation is worth the cost.
Here is the math: if you are paying $2,000 per month in maintenance, a successful termination saves you $24,000 per year — every year until the obligation would have ended. Even a multi-year alimony obligation represents a six-figure exposure.
The cost of a cohabitation investigation — database research, targeted surveillance, and a complete evidence package — is typically a fraction of a single month's maintenance payment. If the evidence supports a motion and the motion succeeds, the return on that investment is immediate and ongoing.
If the evidence does not support a motion, you have saved yourself the cost of a failed legal challenge and the attorney fees that come with it.
A Note on Illinois Jurisdiction
Cook County, DuPage County, Lake County, Will County, and Kane County family courts each have their own procedural norms, but all apply the same Illinois statutory standard for cohabitation. I serve clients throughout the Chicago metropolitan area and have conducted cohabitation investigations in all five collar counties.
If your divorce decree was entered in a different Illinois county or if your ex-spouse has relocated outside the Chicago area, that does not prevent an investigation — it may simply require additional coordination. Contact me and we can discuss the specifics.
The First Step Is a Confidential Conversation
If you are paying alimony in Illinois and believe your ex-spouse may be cohabitating with a new partner, the right move is a confidential consultation — not a premature legal filing.
We will talk through what you know, what the law requires, and what a realistic investigation looks like in your specific situation. There is no obligation and no pressure.
You may be paying money you no longer legally owe. Let's find out.
P.I. Radames is a licensed Illinois private investigator (License #115-002640) serving Chicago and the surrounding suburbs. Military veteran. 20+ years experience. 1,000+ cases closed.
