To facilitate the transition to the hydrogen economy the EU project NATURALHY is studying thepotential for the existing natural gas pipeline networks to transport hydrogen, together with naturalgas, to end-users. Hydrogen may then be extracted for hydrogen fuel-cell applications, or the mixtureused directly by consumers in existing gas-fired equipment, with the benefit of lower carbonemissions. The existing gas pipeline networks are designed, constructed and operated to safelytransport natural gas, mostly methane. However, hydrogen has significantly different properties thatmay adversely affect both the integrity of the network, and thereby increase the likelihood of anaccidental leak, and the consequences if the leak finds a source of ignition. Consequently, a major partof the NATURALHY project is focused on assessing how much hydrogen could be introduced intothe network without adversely impacting on the safety of the network and the risk to the public.Hydrogen is more reactive than natural gas so the severity of an explosion following an accidental leakmay be increased. This paper describes field-scale experiments conducted to measure theoverpressures generated by ignition of methane/hydrogen/air mixtures in a congested but unconfinedregion. Such regions may be found in the gas handling and metering stations of the pipeline networks.The 3 m x 3 m x 2 m high congested region studied contained layers of pipes. The composition of themethane/hydrogen mixture used was varied from 0%2hydrogen to 100%2hydrogen. On the basis of theexperiments performed, the maximum overpressures generated by methane/hydrogen mixtures with25%2(by volume) or less hydrogen content are not likely to be much more than those generated bymethane alone. Greater percentages of hydrogen did significantly increase the explosion overpressure.