The initial stages of the pyrolysis of ethylene have been studied at temperatures in the range 798–924°K and with reactant pressures between 10 and 270 torr. The reaction is a degenerately branched chain process in which the “branching” or secondary initiation step is 1–C4H8→ C3H˙5+ CH˙3 A lower limit to the velocity constant of this reaction has been determined and values have been established for the rate parameters of the reaction, C2H˙5+ C2H4→C2H6+ C2H˙3. The formation of methane and propylene, important products of the reaction, is largely of a secondary nature.