This research guide from the University of Illinois Library, Urbana-Champaign serves as a reference aid for Illinois Statutory Law regarding environmental and pollution regulations, sustainability initiatives, and energy efficiency standards. Commercial groups are encouraged to use guide to meet state requirements.
These are a few free web options for finding Illinois primary authorities.
This Illinois Courts page contains links to full-text opinions issued by the Illinois Supreme Court and the Illinois Appellate Court. Look for the "opinions" links in the Quick Links frame to the left of the screen.
The Illinois General Assembly is Illinois' state legislative body. This portion of its website contains a chapter-by-chapter version of the Illinois Compiled Statutes (ILCS), which contain the statutory laws of Illinos. NOTE: Although very convenient to use for research, this is not considered an "official" version of Illinois' statutes.
This site, from the Illlinois General Assembly's Joint Committe on Administrative Rules (JCAR), contains the best free representation of the Illinois Administrative Code. The Code contains rules and regulations and other materials issued by Illinois administrative agencies.