The state House, in a 115-0 vote, approved a series of changes to a death penalty system that led to the wrongful conviction of at least 17 men.
The action, an override of Gov. Rod Blagojevich's veto, makes the measure law immediately. The Senate overrode the veto earlier this month.
Among other things, the legislation gives the Illinois Supreme Court greater power to throw out unjust verdicts, lets defendants have more access to evidence and bars the death penalty in cases that depend on a single witness.
Lawmakers also approved legislation that would banish police officers who lie in murder investigations, and the governor has agreed to sign it. That issue was the sticking point that led Blagojevich to veto the overall measure.
The overhaul follows years of heated debate over capital punishment, starting with the release from death row of three men in quick succession who were exonerated or found to be wrongly convicted.
In 2000, then-Gov. George Ryan suspended all executions and called on a group of experts to study the issue. And before leaving office last year, he cleared out Illinois' death row, commuting the sentences of 167 prisoners to life in prison.
The new law incorporates most of the expert panel's recommendations.
Under the new law:
Also, the law makes it easier for condemned people to clear their names with newly discovered evidence and guarantees they will be able to see prosecution evidence that favors them - including some previously off-limits documents.
After the release of the Ryan panel's report in early 2002, lawmakers, police and other groups negotiated for more than a year to craft the changes.
Blagojevich had vetoed the measure over the language involving police officers in murder investigations, saying that the punishment process originally proposed did not protect their rights.
By John O'Connor