Hundreds of gallons of oil have leaked into the River Liffey after a road accident involving a tanker and a lorry.
The crash, in Palmerstown , has led to traffic restrictions on a nearby section of motorway.
Two cars were also involved in the incident, but no-one has been seriously hurt.
Dublin Corporation said about 20,000 litres of fuel oil had seeped into the river and efforts are being made to contain the pollution.