The incident happened around 12 p.m. at the Brunch Cafe in the 900-block of Route 22 in Fox River Grove.
Firefighters said an employee dumped a large amount of cleaning chemicals down the drain. As a result, three people inside the restaurant had trouble breathing
A hazmat response team was called and cleaned up the spill.
The restaurant closed for the rest of the day, but is expected to open on time on Tuesday.