The identification of the primary dissociation products of hyperthermal collisions of methane and ethane with a Pt(lll) surface at similar to 150 K and the study of their subsequent thermal evolution has been afforded by the use of reflection-absorption infrared spectroscopy, temperature programmed desorption, and Auger electron spectroscopy. In both cases, the primary dissociation products have been identified as an adsorbed alkyl moiety (methyl, CH3, and ethyl, C2H5, respectively) and an adsorbed hydrogen atom. The thermal treatment of both alkyl adlayers ultimately results in the formation of the ethylidyne (=CCH3) moiety at temperatures between 300 and 400 K. At similar to 500 K, this species itself thermally decomposes to yield adsorbed carbon atoms and gaseous hydrogen. While this behavior may not be unexpected for the C-2 ethyl fragment, the formation of the C-2 ethylidyne fragment from the C-1 methyl fragment can only be explained through the occurrence of carbon-carbon coupling reactions. Such reactions have only recently been observed on the Pt(lll) surface. The C-C coupling reactions associated with the formation of ethylidyne from methyl show considerable dependence upon the surface coverage of the methyl fragment and are accompanied by the evolution of both gaseous hydrogen and methane. The latter results from recombinative desorption of methyl moieties with adsorbed hydrogen atoms.