The cause of the afternoon blaze is still under investigation, said firefighters at the scene.
The fire was reported at 3:10 p.m. The structures were in the area of Broadway Avenue and 2nd Street. People in the area were evacuated, a dispatcher said.
Firefighters at the scene described the structures as outbuildings. Ruth Parsons, owner of the nearby Boise Foundry & Machine in Melba, said she thought the fire had burned in old houses that were being used as sheds.
"They're not very far from TK's Oil facility," she said. "They still have trucks down around TK Oil."
A call to TK Oil Tuesday afternoon went unanswered.
Parsons said the fire might have been much worse if firefighters hadn't responded so quickly.