The upper flammability limit (UFL) of methane/hydrogen/air mixtures is determined for hydrogen fuel molar fractions of 0%, 20% and 40%, at initial pressures up to 10 bar and initial temperatures up to 200 degrees C. The experiments are performed in a closed spherical 4.2dm(3) vessel. The mixtures are ignited by fusing a coiled tungsten wire, placed at the centre of the vessel. A 5% pressure rise criterion is used to determine the flammability limits. The results show the UFL to increase with increasing initial pressure, initial temperature and hydrogen fuel concentration. At atmospheric pressure a second experimental set-up is used, to allow comparison between the bomb and tube method as described by European standard EN 1839. It is found that with increasing initial temperature the pressure rise criterion used in the bomb method becomes less sensitive than the visual flammability criterion used in the tube method. (C) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.