Scottish Fire and Rescue Service was called out to the DoubleTree by Hilton in Dundee at around 7.30am yesterday.
Six vehicles, including two specialist appliances, were at the scene.
The premises were later handed back to hotel management after the incident had been dealt with by firefighters.
A fire service spokeswoman said: "It was a chemical reaction, a reaction between two chemicals in the leisure area of the hotel.
"The area has been ventilated and the chemicals have been isolated.
"Firefighters in gas-tight suits and breathing apparatus dealt with it.
"The hotel was fully evacuated."
Nobody is thought to have been injured in the incident.