In the thermal decomposition of methane at temperatures from 880 to 1103 K, hydrogen and ethane are the only primary products. The rate of formation of ethane falls rapidly towards zero as the reaction progresses until ethane reaches a steady-state concentration. This behaviour is interpreted in terms of a radical chain mechanism,[Formula: see text]Values of k4 were obtained which confirm the non-Arrhenius behaviour of this reaction at these temperatures. Similar chain sequences propagated by addition or abstraction reactions of methyl radicals with ethylene, propylene, and acetylene can account for the formation and disappearance of these secondary products.At a later stage in the pyrolysis a marked autocatalysis is observed and the yield of ethane increases sharply above its steady-state value. It is concluded that this autocatalysis is largely a homogeneous process and is not caused by or associated with carbon formation. Deposition of carbon on the surface was observed at a still later stage of the decomposition, and was quantitatively estimated by light absorption measurements. Possible mechanisms for the auto catalysis are discussed.