To facilitate the transition to the hydrogen economy the EU project NATURALHY is studying the potential for the existing natural gas pipeline networks to transport hydrogen, together with natural gas, to end-users. Hydrogen may then be extracted for hydrogen fuel-cell applications, or the mixture used directly by consumers in existing gas-fired equipment, with the benefit of lower carbon emissions. The existing gas pipeline networks are designed, constructed and operated to safely transport natural gas, mostly methane. However, hydrogen has significantly different properties that may adversely affect both the integrity of the network, and thereby increase the likelihood of an accidental leak, and the consequences if the leak finds a source of ignition. Consequently, a major part of the NATURALHY project is focused on assessing how much hydrogen could be introduced into the 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 leak may be increased. This paper describes field-scale experiments conducted to measure the overpressures generated by ignition of methane/hydrogen/air mixtures in a congested but unconfined region. 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 the methane/hydrogen mixture used was varied from 0%2hydrogen to 100%2hydrogen. On the basis of the experiments performed, the maximum overpressures generated by methane/hydrogen mixtures with 25%2(by volume) or less hydrogen content are not likely to be much more than those generated by methane alone. Greater percentages of hydrogen did significantly increase the explosion overpressure.