Sirens at Imperial Oil's Sarnia manufacturing site sounded just after 11:30am Saturday.
Operations Manager Chris Jessup said plant crews responded to a vapour release at the refinery.
The company reported the leak, in Area 2, was isolated and depressurized within about a half hour and the all clear issued.
Winds at the time were south-southwest 24 km/h gusting to 41 km/h.
Monitoring at the plant perimeter showed no measurable contaminants according to the company.
There were no injuries.
The Environment Ministry was notified.
