We used the B3LYP flavor of density functional theory (DFT) to study the chemisorption of all CH, and C2Hy intermediates on the Pt(111) surface. The surface was modeled with the 35 atom Pt-14.13.8 cluster, which was found to be reliable for describing all adsorption sites. We find that these hydrocarbons all bind covalently (sigma-bonds) to the surface, in agreement with the studies by Kua and Goddard on small Pt clusters. In nearly every case the structure of the adsorbed hydrocarbon achieves a saturated configuration in which each C is almost tetrahedral with the missing H atoms replaced by covalent bonds to the surface Pt atoms. Thus, (Pt-3)CH prefers a mu(3) hollow site (fcc), (Pt-2)CH2 prefers a mu2 bridge site, and PtCH3 prefers mu(1) on-top sites. Vinyl leads to (Pt-2)CH-CH2(Pt), which prefers a mu3 hollow site (fee). The only exceptions to this model are ethynyl (CCH), which binds as (Pt-2)C=CH(Pt), retaining a CC pi-bond while binding at a mu(3) hollow site (fcc), and HCCH, which binds as (Pt)HC=CH(Pt), retaining a g bond that coordinates to a third atom of a mu(3) hollow site (fcc) to form an off center structure. These structures are in good agreement with available experimental data. For all species we calculated heats of formation (DeltaH(f)) to be used for considering various reaction pathways on Pt(111). For conditions of low coverage, the most strongly bound CH. species is methylidyne (CH, BE = 146.61 kcal/mol), and ethylidyne (CCH3, BE = 134.83 kcal/mol) among the C2Hy molecules. We find that the net bond energy is nearly proportional to the number of C-Pt bonds (48.80 kcal/mol per bond) with the average bond energy decreasing slightly with the number of C ligands.