The pyrolysis of mixtures of methane and ethylene has been studied over the temperature range 774–853 K at total pressures below atmospheric to explore the effectiveness of ethylene as a catalyst for conversion of methane to higher hydrocarbons. Pyrolysis of ethylene alone at these temperatures yields ethane, propylene, butene-1, and butadiene as the major volatile products. In the lower temperature region the addition of methane increased the rate of production of ethane through the chain propagation reaction,[Formula: see text]thus effecting the conversion of methane to ethane. The rates of formation of butene-1 and butadiene were not appreciably altered and that of propylene was increased only at low pressures of ethylene. At higher temperatures methane caused a substantial increase in the rate of production of propylene, the result of the increasing importance of the methyl radical in the propagation reactions. Thus ethylene can efficiently convert methane to ethane or to propylene depending on the temperature of the reaction. Addition of small quantities of butene-1, which acted as a secondary initiator, to the mixtures increased considerably the overall rate of the conversion of methane. A mechanism is discussed which accounts for the main features of the reactions. Keywords: methane, ethylene, kinetics, pyrolysis, catalysis.