Microcalorimetric measurements of C2H4 adsorption on Pt powder at 303 and 173 K gave initial heats of 160 and 120 kJ/mol, which correspond to formation of ethylidyne species plus atomic hydrogen and to molecularly bonded C2H4 species, respectively. Microcalorimetric measurements of C2H2 adsorption on Pt powder at 303 and 173 K gave initial heats of 220 and 210 kJ/mol, respectively. Initial heats of H-2 adsorption on Pt powder at 403 and 173 K were 90 kJ/mol. Exposure of Pt samples to C2H4 hydrogenation at 298 K under conditions for which ethylidyne species form, results in decreases in the saturation coverages of H(2 )and C2H4 adsorption, as well as decreases in the strengths of interaction of these species with the surface (by 20-30 kJ/mol) at 173 K. Exposure of Pt to C2H4 hydrogenation at conditions for which ethylidyne species do not form, results in a decrease in the saturation uptake and the adsorption strength of H-2 at 173 K. These surfaces contain significant amounts of adsorbed hydrogen, since they show formation of C2H6 upon C2H4 adsorption at 173 K.