Studies have been made of the pyrolysis of vinylacetylene in the temperature range 300-620°C. At low temperatures the reaction is essentially a second-order polymerization process with an activation energy of 28 kcal mol–1. Above 500°C, however, the pyrolysis is accompanied by the formation of large amounts of low-molecular-weight products, particularly hydrogen and acetylene. A mechanism has been proposed which accounts for the principal features of the pyrolysis of vinylacetylene and it is suggested that an analogous mechanism may be operative during the pyrolysis of acetylene also.