The adsorption geometry and diffusion behavior of ethylidyne (CCH3) on Pt(111) is studied in order to help elucidate the role of this hydrocarbon species during ethylene hydrogenation and dehydrogenation over Pt catalysts. A variant of the extended Huckel method is used, which allows bond-specific parametrization in molecules. It is adapted to geometrically infinite periodic systems, and empirical parameter values suitable for hydrocarbons on Pt surfaces are developed. The 3-fold fee and hcp hollows are found to be the most stable adsorption sites, with an energy barrier of about 0.11 eV between them. This barrier suggests relatively easy diffusion of ethylidyne along the Pt(111) surface (subject only to steric intermolecular constraints), allowing unimpeded ethylene approach by opening up sites where subsequent hydrogenation reactions could take place with preadsorbed hydrogen.