A series of hydrogen fuel cell powered cars will arrive in Cardiff today, as part of promotional drive by manufacturers to increase public awareness of the technology.
The vehicles - made by Daimler, Hyundai, Honda and Toyota - are marketed as a cleaner, more efficient alternative to current cars.
The cars run on electricity, derived from hydrogen fuel cells. Unlike some electric vehicles, the power is generated on board by the cells, instead of being contained in batteries.
Inside the cells, hydrogen is combined with oxygen to create electricity to drive the vehicle's electric motor.
The tour, which is visiting nine European cities, is a bid by the manufacturers to raise the profile of the vehicles with politicians and the public.
In Cardiff and London, people will be able to test-drive the cars. In Swindon, where some of the cars are manufactured, there will also be a refueling demonstration.
The companies behind the vehicles say they want to see the development of refueling facilities across the UK in order to make the vehicles viable for customers.
The event will take place outside Cardiff's Millennium Centre throughout the day.