A minor oil spill in the River Avon at Pewsey on Saturday is being investigated by Wiltshire Council.
It is believed that the substance is heating oil which is causing a sheen on the water in the centre of the village.
The Environment Agency is providing support to the council during the investigation.
Paul Gainey, of the Environment Agency, said: “We deployed a boom to recover some of the oil.
“Quite a few people have noticed it but it looks worse than it is. There are no reports of dead fish.
“We’ll continue to monitor the river and replace the boom as and when necessary.”