A security guard suffered serious facial injuries after being sprayed with what is believed to have been an ammonia-type substance outside a bank.
The man was left "very shaken" by the attack in West Yorkshire, during which the suspect, thought to be aged in his 20s, made off with the cash box the security guard had been holding.
No-one has been arrested following the cash in transit robbery shortly after 1pm on Tuesday outside Barclays Bank in Brighouse, Calderdale.
The victim, who has been taken to Calderdale Royal Infirmary for treatment, is an employee of security firm G4S.
Area risk consultant for G4S Cash Solutions, Duncan Watson, described the attack as "violent" and vicious".
He said: "On Tuesday 23 January, one of our cash crews was the victim of a violent attack during a routine service on Briggate, Brighouse. Our employee has suffered serious facial injuries and is currently receiving treatment in hospital. He has understandably been left very shaken by this vicious attack.
"We are working closely with West Yorkshire Police and I would urge anyone who may have been nearby and seen this attack to get in touch with the police or dial 101 so that those behind this attack can be brought to justice."