Illiopolis is a village in Sangamon County, Illinois, United States.

[3] According to the 2010 census, Illiopolis has a total area of 0.46 square miles (1.19 km2), all land.

[4] As of the census[6] estimates of 2011, there were 897 people, 362 households, and 263 families residing in the village.

[7] On April 24, 2004, the Formosa Plastics plant (formerly Borden) in Illiopolis exploded, killing five people, injuring others and causing a portion of the town to be evacuated.

On March 6, 2007, the United States Chemical Safety Board (CSB) released its final report on the incident indicating that an operator bypassed a safety interlock and drained a pressurized reactor instead of an empty one they were in the process of cleaning.

The report listed as root causes, that Formosa-IL and Borden Chemical "did not adequately address the potential for human error.