Tesoro Corp said two workers were injured following a chemical release at its 166,000-barrel-per-day (bpd) Golden Eagle refinery in Martinez, California, on Wednesday.
The company said it has shut a gasoline production unit at the refinery following the incident.
"The cause of the incident is under investigation," company spokeswoman Tina Barbee said, adding that the two employees, who were exposed to the chemical release, have been transported to the UC Davis Medical Center.
The San Francisco Chronicle reported on its website that the workers were splashed with acid.
The company, one of California's biggest gasoline producers, houses a 73,000-bpd gasoline-making fluid catalytic cracker (FCC) unit at the refinery, according to IIR Energy data available via Eikon.
There was a minor fire at the refinery on February 9 during planned maintenance, and the company said on Tuesday that there was no operational impact due to the incident.
The Golden Eagle refinery, also known as the Avon refinery, produces motor fuels, including cleaner-burning California Air Resources Board (CARB) gasoline and CARB diesel fuel, as well as conventional gasoline and diesel fuel, according to its website.
(Reporting by Arpan Varghese and Nallur Sethuraman in Bangalore; Editing by Jeffrey Benkoe and Marguerita Choy)