About 30 firefighters and five fire engines were called to the Clynelish Distillery near Brora on the east coast of Sutherland on Tuesday evening.
The Scottish Fire and Rescue Service (SFRS) said there had been an accidental mixing of two substances that resulted in the production of chlorine gas.
It later emerged that the incident had been at the distillery's cooling tower, and not inside the plant where Clynelish Single Malt Whisky is produced.
One man was taken to Raigmore Hospital in Inverness for treatment suffering from breathing difficulties. He was kept in overnight for observation.
Firefighters in gas- tight suits and breathing apparatus isolated the tank and made the area safe prior to the arrival of a specialist clean-up company.
A spokesman for Diageo, which owns the distillery, said: "There was an incident involving a contractor working at the cooling tower at the distillery.
"The health and safety of our staff and our contractors is paramount and we took the necessary action under the circumstances to ensure the wellbeing of the contractor. He was taken to hospital as a precaution and has now been discharged is returning to work today."
SFRS Group Manager John MacDonald said: "This was an incident with potentially serious outcomes, but due to the quick thinking of on-site staff at the distillery and the response from the emergency services, the incident was dealt with quickly and effectively."