Guide
The Illinois domicile test, and how a move actually gets proved
Updated
Illinois has no exit tax. The only risk is that, in its view, you did not leave. That turns on facts, easier arranged before the move than after a notice.
Question 1
Which state are you leaving?
The mechanism differs completely by state, and the phrase exit tax describes different things in each.
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The definition
The Illinois Department of Revenue states that you are an Illinois resident if you were domiciled in Illinois for the entire tax year.
Your domicile is the place where you reside and the place where you intend to return after temporary absences.
Both halves matter. Reside is about where you actually are. Intend to return is about what you plan, and planning is invisible, so it gets inferred from everything else.
What gets read
Where the home you actually live in is, and what happened to the previous one. Where the family is and where children are at school. Driving licence and vehicle registration. Voter registration. Professional licences. The doctor, the dentist and the vet. Safe deposit boxes and club memberships. The address on the employer's payroll record.
No single item settles it, which is exactly why the pattern is what matters. A set of small facts all pointing one way is far more persuasive than one dramatic one pointing the other.
The date
A part-year return splits the year at the change of domicile, so the date is doing real work and is almost never evidenced at the time.
Pick it deliberately and attach something contemporaneous: the closing on the old home, the lease start on the new one, the day the family actually moved. Then use it consistently everywhere, because inconsistent dates across a return, a licence and an employer record are the kind of small discrepancy that invites a longer look.
Second homes
Keeping property in Illinois does not by itself prevent a change of domicile. Keeping property and continuing to use it as a home is a different matter, and the distinction is behavioural rather than legal.
If the Illinois house is retained, the useful test is whether an outside reader looking only at how it is used would call it a second home or a home. Renting it out answers the question cleanly. Keeping it available and furnished, and staying in it regularly, does not.
How to terminate Illinois residency
You end it by changing your domicile. Illinois treats you as resident if domiciled there for the entire tax year, and domicile is where you reside and intend to return after temporary absences.
So move your actual home to another state, with no intention of returning to live. Then make the record say so: the licence, vehicle registration, voter registration, doctors and the address your employer holds.
For the year you move you file a part-year return, splitting income at the date your domicile changed, and Illinois keeps taxing Illinois-source income after that date. There is no form that terminates residency on its own and no charge for leaving; the part-year return and the record behind the date are what prove the move.