Environment Agency Wales is appealing for information after fuel was spotted in the river Rhymney in Machen.
Officers are currently on site following a report from a member of the public, and are attempting to trace the source of the pollution.
Emily Roe, Environment Agency Wales, said: “Despite a heavy visual sheen, our initial investigations suggest there is little environmental impact on the river.
“We are appealing for information from the public and urge them to contact our Incident Hotline on 0800 80 70 60. All calls will be treated in strictest confidence.”