The chemistry of dimethyl mercury on a Pt(lll) single crystal surface has been investigated by reflection-absorption infrared spectroscopy (RAIRS). Dimethyl mercury appears to be highly reactive on Pt(lll) and readily decomposes on the surface at temperatures of 100 g and above. Adsorption at 100 K initially occurs in a dissociative manner to produce CH3 and CH3Hg species on the surface, both of which are identified as having C-3v local symmetry. At higher exposures, molecular adsorption dominates with the Hg-C-Hg axis initially oriented parallel to the surface. This preferred orientation, however, does not persist into the multilayer. Thermal treatment of the surface layer results in multilayer desorption between 130 and 135 K, and no parent molecular species are observed beyond 160 K. Adsorption at 200 and 300 K produces an overlayer consisting primarily of CH3Hg species, which are thermally stable to about 350 K. Subsequent heating to 400 K results in the formation of ethylidyne species which are characterised by RAIRS. Adsorption at 400 K results in the direct formation of an ethylidyne layer estimated to be about 85% saturated coverage.