It happened during a robbery outside a bank in the town on Tuesday afternoon.
A security guard is in hospital after being attacked with a substance in Brighouse.
He was collecting money from a bank on Briggate in the town just after 1pm this afternoon when the liquid was sprayed in his face.
The robber got away with the cash box and the victim was taken to Calderdale Royal Infirmary.
The substance has yet to be identified.
A West Yorkshire Police spokesperson said:
"Enquiries are ongoing in Brighouse town centre by Calderdale CID.
Anyone who witnessed the incident is asked to contact Calderdale CID on 101 referencing police log 0870 of 23 1 2018."