A series of large scale vapour cloud explosions in a long congested region were conducted using methane/hydrogen mixtures. The congested region measured 3m x 3m x 18 m long and was preceded by a confined region which allowed an explosion flame with some initial flame speed and turbulence to be generated which then entered the congested region. During the experiments the flame speed and explosion overpressure were measured through the congested region. The hydrogen content in the methane/hydrogen mixture was varied from 0 to 50% by volume. A key objective was to determine factors that could lead to continued flame acceleration through the congested region, such as the hydrogen concentration, the initial flame speed entering the congestion and the level of congestion. The results are reported together with some detailed observations of the complex nature of pressure traces produced by explosion events of this type. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.