A 27-year-old man had an "unknown liquid" thrown in his face while walking on a Borders pathway.
The incident happened at 06:45 on the Waterside Walkway next to the A68 in Jedburgh.
The victim was taken to Borders General Hospital as a precaution and was subsequently released.
Police said they had established the liquid was not a corrosive substance but inquiries were ongoing to identify it.
Det Sgt Callum Peoples said: "This was a frightening experience for the victim who was thankfully not seriously injured as a result.
"I am eager to hear from anyone who may have been in the Bongate area of Jedburgh and who saw anyone acting suspiciously, or who witnessed what happened, as soon as possible.
"The victim was passed by an Alsatian-type dog immediately prior to the assault and I would appeal to its owners to come forward to assist with our investigations."