We investigate changes in the combustion behavior of methane, the primary component of natural gas, upon hydrogen addition by characterizing the autoignition behavior of methane/hydrogen mixtures in a rapid compression machine (RCM). Ignition delay times were measured under stoichiometric conditions at pressures between 15 and 70 bar, and temperatures between 950 and 1060 K; the hydrogen fraction in the fuel varied between 0 and 1. The ignition delay times in methane/hydrogen mixtures are well correlated with the ignition delay times of the pure fuels by using a simple mixing relation reported in the. literature. Simulations of the ignition delay times using various chemical mechanism are also reported. The mechanism given by Petersen et al. shows excellent agreement with the measurements for all mixtures studied. Initial results on fuel-lean mixtures show a modest effect of equivalence ratio on the delay times. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.