In this paper the transient process leading to ignition of a combustible mixture of methane/ air is studied, using numerical techniques for stiff differential equations driven by the Computational Singular Perturbation (CSP) routine in order to obtain the appropriate reduced kinetic mechanism for high temperature, atmospheric pressure mixtures. A minimum of six global steps are obtained from 231 elementary chemical reactions taken into account. Main products during the ignition process are CO, C 2 H 6 , C 2 H 4 , C 2 H 2 , H 2 , H 2 O and very small amounts of CO 2 . Radicals CH 3 and CH 2 O are not in steady state at the beginning. However, these species reach the steady-state conditions at times much shorter than the ignition delay time, thus it can be assumed to be always in steady-state. The numerical results obtained from detailed chemistry compare very well with the results obtained with the six-step reduced mechanism, for initial temperatures higher than 1300 K. Also, a good comparison is obtained with experimental results.