Electric buses powered by hydrogen fuel cells are now a regular sight on the RV1 bus route which runs along the SE1 riverside between Waterloo Bridge and Tower Bridge.
Non-fuel cell Mercedes Citaro buses outside Tate Modern at the RV1's launch in 2002.
The vehicles are in service between 7.30am and 2.30pm on weekdays.
The fuel cell vehicles are similar to the normal Mercedes Citaro vehicles which operate on the RV1 route, but have roof-mounted tanks which store hydrogen.
Nine cities in Europe are taking part in the fuel cell bus trial, making it the largest project of its type anywhere in the world.
The only emission from a fuel cell bus is water, which forms a vapour cloud as soon as it leaves the exhaust and enters the atmosphere.