As part of a continuing program focused on the role of hydrogen in catalytic reactions, cyclopropane hydrogenation on the Pt(lll) surface has been characterized using in situ soft X-ray studies above the carbon K edge. In situ soft X-ray methods provide interesting new information regarding concentrations, stoichiometries, bonding, and reactivities of adsorbed carbon-containing species under reaction conditions. Ar low temperature, cyclopropane: is weakly adsorbed and tilted up from the Pt(lll) surface. The saturation coverage is 4.4 x 10(14) molecules/cm(2) at 100 K. Catalytic hydrogenation of cyclopropane to form propane is observed during batch reactivity studies in the 350 K range. No methane or ethane products are observed. Approximately 2.9 x 10(14) C-3 molecules/cm(2) of adsorbed carbonaceous species are observed on the surface at 350 K under reaction conditions. The concentration of these species decreases above 350 K in excess hydrogen. In situ isothermal reactivity studies in hydrogen near 350 K indicated chat a significant fraction of these species can be removed from the surface with a thermal activation energy of 15.2 kcal/mol. Taken together the observation of catalytic propane formation and the estimated activation energy suggests that the surface species are directly involved in propane formation. In situ characterization of this species, using soft X-ray C-H intensities to determine stoichiometry, indicates that a C3H6 species is dominant up to 320 K. In situ glancing and normal spectra taken at 320 K indicate that the C3H6 species is a platinacylobutane intermediate adsorbed in an upright configuration relative to the surface.;Increasing temperature to 350 K under reaction conditions increases the average C-H stoichiometry to C3H7 This hydrogen addition suggests formation of adsorbed propyl in the 350 K temperature range. Taken together these experiments indicate that the dominant mechanism for C-C bond breaking is associated with insertion of the Pt surface into the adsorbed cyclopropane reactant to form a metallocycle intermediate. This metallocycle is strongly bound and stable up to 320 K in large excesses of hydrogen. With increasing temperature this C-3 platinacyclobutane intermediate is hydrogenated to form propane in the 350 K range. Observation of a hydrogenated C3H7 intermediate suggests that propane formation may involve sequential hydrogen addition and a transient propyl intermediate.