A man has been arrested after a trick-or-treater dressed as a ghost threw corrosive liquid over a man.
The victim suffered burns to his face and feet in the attack in High Wycombe at about 19:45 GMT on 31 October.
He answered the door to a trick-or-treater dressed in a white sheet with "eye holes cut out", police said.
A 33-year-old man from High Wycombe has been arrested in connection with the offence.
Thames Valley Police said he had been released under investigation.
He was arrested on suspicion of causing grievous bodily harm with intent and throwing or casting a corrosive fluid with intent to burn, maim, disfigure, disable or do grievous bodily harm.
The victim was treated at High Wycombe General Hospital and later discharged.