The sirens sounded at the Sarnia Imperial Oil refinery at 4:10am Thursday morning.
The company say a small fire in a processing unit was extinguished within 45 minutes and the all clear issued at 5:15am.
The company says no injuries were reported in the incident and a Chemical Valley Emergency Coordinating Organization alert was initially issued.
Officials say monitoring at its fence-line recorded no impact to air quality and all regulatory authorities have been notified including the Ministry of Environment and Climate Change.