Two men were taken to hospital with serious burns after a substance was thrown at them, police said.
West Midlands Police said the potentially hazardous substance was thrown just before 10:30 BST while the men were sitting in a van on Wollaston Road in Stourbridge.
The road was closed following a minor crash which resulted after the attack, police added.
The force said its inquiries were continuing.
It also appealed for anyone with information to come forward.
West Midlands Fire Service also confirmed it attended the scene with specialist officers and equipment.
Follow BBC West Midlands on Facebook, on Twitter, and sign up for local news updates direct to your phone.