Four suspected acid attacks took place in less than 90 minutes as two men on a moped carried out a string of robberies, police said.
One victim was said to have suffered "life-changing" injuries during one of the attacks in east London.
The Metropolitan Police said the incidents appeared to be linked and two involved victims having their mopeds stolen.
The attackers pulled up to the four separate men and doused them the corrosive liquid between 10.25pm and 11.37pm, police said.
At the start of the spree, a 32-year-old moped driver had been approached by the pair as he drove on the Hackney Road junction with Queensbridge Road.
The two male suspects had tossed the noxious substance into his face before one of them jumped on to his vehicle and drove away.
Police said the man had gone to an east London hospital and they were awaiting an update on his injuries.
Around 40 minutes later, at around 11.05pm, another victim was attacked by two men on a moped on Shoreditch High Street, having liquid thrown in his face.
His injuries were not life-threatening, police said.
Within 15 minutes the attackers appear to have struck again, launching the corrosive substance at a man on Cazenove Road, causing "life-changing" injuries.
The final assault of the night was reported to police at 11.37pm, when another man was confronted as he sat on his moped in traffic on Chatsworth Road.
After again spraying the liquid in a victim's face, the moped was stolen and both attackers fled.
No arrests have been made.
The Met Police said in a statement: "Inquiries are ongoing and officers from Hackney are currently trying to trace the suspects concerned."