Several features of the pyrolysis of acetylene in the temperature range 500 to 1000 $^\circ$C suggest that the underlying mechanism is the same as that in operation at both lower and higher temperatures. Thus the rate of consumption of acetylene obeys a second-order kinetic law and is little affected by additions of the gaseous products; the observed velocity constants agree closely with those expected if a single mechanism were in operation over the whole temperature range 350 to 2500 $^\circ$C. The analytical results show conclusively that the sole initial product of the reaction is vinylacetylene. Diacetylene appears to be formed by heterogeneous decomposition of vinylacetylene and this reaction can occur at ambient temperatures in the presence of carbonaceous deposits. Methylacetylene and benzene probably arise from the further reaction of C$_6$ species which are formed during the building-up of polymer chains. Methane is the predominant final gaseous product of the reaction at the temperatures investigated. A mechanism is proposed which accounts for both the chain character of the reaction and the apparent existence of equilibria. It appears that acetylene is first converted to the triplet state by a surface reaction. This initial stage is then followed either by the production of C$_4$ and subsequent species in the triplet state by a series of step-wise reactions involving further acetylene or by the regeneration of stable species as a result of reaction, at a surface, of the corresponding excited species.