An elderly man almost lost his sight in one eye after being sprayed with acid by two men who stole his Cartier watch.
Police are now appealing for witnesses to the “despicable, senseless” attack.
Were it not for “swift intervention and treatment at the scene”, his sight could have been badly damaged.
The 75-year-old told officers he had been walking along a road in Stanmore, northwest London, when the suspects rode towards him on a scooter just after 5.30pm on Wednesday.
They forcibly grabbed his wrist and stole his watch, he said, before one of them squirted a “noxious substance” at his face from a bottle.
The victim, who was cared for by a passing doctor and an ambulance crew, did not need to be taken to hospital.
The scooter is described as “completely black”, while the suspects were in all-black clothing made of “canvas/tactical material”.
Visors on their helmets covered their faces entirely.
The stolen watch is a Cartier Pasha with a brown leather strap, gold rim and gold face. It is customised and engraved with hour numbers.
“This is a despicable attack on an elderly gentleman going about his daily business,” said DS Amar Patel.
“The victim was unable to defend himself against this vicious robbery which was followed up by a senseless attack with what we now know to be a noxious substance.”
DS Patel added: “A specialist has confirmed that were it not for the swift intervention and treatment he received from people who attended to him at the scene, he may well have lost his sight in one eye.
“It is vital that we bring the culprits to justice before they target anyone else.”
Anyone with information should call police on 07769 162964, or Crimestoppers, anonymously, on 0800 555111. Officers can also be tweeted via @MetCC.