Nitric acid has leaked into the river Rhine from a barge berthed at Bayer's plant at Krefeld, Germany.
The leak sent out fumes and caused a fire on board the barge.
Residents in the Uerdingen district were warned to keep their windows and doors closed because of the fumes.
But Bayer says no one was hurt, and checks for airborne contamination showed no danger for humans.
A statement from Bayer's Krefeld-Uerdingen plant says the barge was carrying some 1,800 tonnes of nitric acid, which is used in the production of polyurethane.