Quick answer: exclusion of $4,000,000, with a genuine cliff above it — but the exact dollar amount above the exclusion comes from an "interrelated calculation," not a table you can read off. Get a calibrated estimate in the Illinois estate tax calculator.
Why the state built its own calculator instead of a table
The verified official examples
The AG's own fact sheet publishes exact reference points, reproduced here directly: a $4,000,000 estate (all Illinois property) owes $0. A $5,000,000 estate (all Illinois property) owes $285,714. The same $5,000,000 estate, split 50% Illinois and 50% Florida, owes exactly half — $142,857 — confirming the apportionment method described below.
How multi-state estates get apportioned
For both resident and nonresident decedents, a preliminary tax is calculated first assuming all assets sit in Illinois. That figure is then multiplied by the ratio of Illinois-situs assets to total assets to reach the final apportioned tax — exactly what the 50/50 Illinois-Florida example demonstrates, cutting the tax precisely in half.
Filed with the Attorney General, paid to the Treasurer
Unusually among the states, Illinois's estate tax is administered by the Attorney General's office rather than a department of revenue. Form 700 is filed with the AG's office (in Chicago or Springfield, depending on county), while the tax itself is paid directly to the Illinois State Treasurer.
A local probate attorney can review your estate — many offer a free consultation.
The return is due 9 months after death, matching the federal deadline, with extensions available through the Attorney General's office.