| | | Peoria County | IL | 17143 | | Peoria, IL 37900 | Peoria-Canton, IL 427 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=PeoriaCounty+IL |
| | | Rock Island County | IL | 17161 | | Davenport-Moline-Rock
Island, IA-IL 19340 | Davenport-Moline, IA-IL 209 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=RockIslandCounty+IL |
| | | St. Clair County | IL | 17163 | | St. Louis, MO-IL 41180 | St. Louis-St.
Charles-Farmington, MO-IL 476 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=St.ClairCounty+IL |
| | | Sangamon County | IL | 17167 | | Springfield, IL 44100 | Springfield-Jacksonville-Lincoln,
IL 522 | Central | metro county (urban) | federal/state
government-dependent | https://maps.google.com/?q=SangamonCounty+IL |
| | | Stephenson County | IL | 17177 | | Freeport, IL 23300 | Rockford-Freeport-Rochelle,
IL 466 | Central | nonmetro county (rural) | manufacturing-dependent | https://maps.google.com/?q=StephensonCounty+IL |
| | | Tazewell County | IL | 17179 | | Peoria, IL 37900 | Peoria-Canton, IL 427 | Central | metro county (urban) | manufacturing-dependent | https://maps.google.com/?q=TazewellCounty+IL |
| | | Vermilion County | IL | 17183 | | Danville, IL 19180 | Champaign-Urbana-Denville,
IL 169 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=VermilionCounty+IL |
| | | Whiteside County | IL | 17195 | | Sterling, IL 44580 | Dixon-Sterling, IL 221 | Central | nonmetro county (rural) | manufacturing-dependent | https://maps.google.com/?q=WhitesideCounty+IL |
| | | Will County | IL | 17197 | Chicago-Naperville-Schaumburg,
IL 16984 | Chicago-Naperville-Elgin,
IL-IN 16980 | Chicago-Naperville,
IL-IN-WI 176 | Central | metro county (urban) | nonspecialized | https://maps.google.com/?q=WillCounty+IL |
| | | Williamson County | IL | 17199 | | Marion-Herrin, IL 32060 | Carbondale-Marion-Herrin,
IL 166 | Central | metro county (urban) | federal/state
government-dependent | https://maps.google.com/?q=WilliamsonCounty+IL |
