The thermally induced rearrangements and reactions of acetylene on the (111) crystal face of platinum were observed by monitoring the vibrational spectra of surface adsorbates in the CH stretch range with infrared-visible sum frequency generation. It was found that adsorption of acetylene at 125 K led directly to the formation of an adsorbed vinylidene species (eta(2)-mu(3)-CCH2). Upon annealing, a variety of different species were formed that included ethylidene (M=CHCH3), di-sigma-bonded ethylene (M-CH2CH2-M), mu-vinylidene (M=CCH2), and ethylidyne (M=CCH3). The presence of species with C2H4 and C2H3 stoichiometry was strong evidence that a disproportionation reaction must occur. Above 381 K ethylidyne was the only observable species on the surface.