The Delaware Department of Natural Resources and Environmental Control says the Delaware City Refinery released 500 pounds of sulfur dioxide.
The incident occurred late last night for the second time in August.
The cause is still under investigation.
State officials say the company released the chemical into the air in early August.
In that instance it was due to an equipment malfunction that has since been fixed.
The Environmental Protection Agency has classified Sulfur dioxide as a hazardous substance.