A man suffered serious burns when a liquid was thrown in his face by three youths when he refused to give them a cigarette.
The incident happened on Monday afternoon in Hornbrook Gardens, Plymouth, Devon.
The 45-year-old victim is being treated in Derriford Hospital, but he may have to be transferred to a specialist burns unit in Bristol.
Police said the substance was being tested.
Det Insp Mike Cooper told BBC News that it appeared to be alkaline-based liquid.
"This was a serious and very nasty assault which has left a man with potentially life-changing injuries," he said.
"It is essential that we establish exactly what happened and deal with whoever is responsible as quickly as possible."
House-to-house inquiries are being carried out in the Southway area, but anyone with information about the assault is asked to contact police.