Firefighters were called to Monument Underground Station 7.40am.
The station was closed as the fire service made the scene safe.
Station Manager, Daniel Alie, who was at the scene, said: "Firefighters carried out tests and found a small amount of ammonia had leaked in a basement storeroom.
The ammonia wasn’t at a harmful level. We're now satisfied that the area is safe and have handed the incident back to London Underground Staff."
Both Monument and Bank underground stations were evacuated as a precaution. Bank station is now open. Firefighters will be leaving the scene soon.
Firefighters were called called at 7.47am and the incident was closed at about 10.30am.
City Police were also on the scene to marshall commuters through the chaos.