Imperial personnel responded to what the company called a small leak of light hydrocarbon material at the Sarnia refinery late Monday morning.
Company spokesman Jon Harding says the release from a processing unit occurred at about 10:55am.
He says there were no injuries and precautionary air quality monitoring at the fence line did not identify any issues of concern.
Plant emergency warning sirens were activated to alert personnel to respond to the issue.
The all clear was issued at about 12:30pm.
