If the general public is to use hydrogen as a vehicle fuel, customers must be able to handle hydrogenwith the same degree of confidence, and with comparable risk, as conventional liquid and gaseousfuels. The hazards associated with jet releases from leaks in a vehicle-refuelling environment must beconsidered if hydrogen is stored and used as a high-pressure gas since a jet release in a confined orcongested area could result in an explosion. As there was insufficient knowledge of the explosionhazards, a study was initiated to gain a better understanding of the potential explosion hazardconsequences associated with high-pressure leaks from refuelling systems. This paper describes twoexperiments with a dummy vehicle and dispenser units to represent refuelling station congestion. Thefirst represents a ?worst-case? scenario where the vehicle and dispensers are enveloped by a 5.4 m x6.0 m x 2.5 m high, pre-mixed, hydrogen-air cloud. The second is an actual high-pressure leak fromstorage at 40 MPa (400 bar), representing an uncontrolled, full-bore, failure of a vehicle refuellinghose. In both cases an electric spark ignited the flammable cloud. Measurements were made of theexplosion overpressure generated, its evolution with time, and its decay with distance. The resultsreported provide a direct demonstration of the explosion hazard from an uncontrolled leak; they willalso be valuable for validating explosion models that will be needed to assess configurations andconditions beyond those studied experimentally.