We have investigated the thermodynamics of the dehydrogenation of propylene to propylidyne on Pt(111) for a 0.25 ML coverage. We have also determined the adsorption energies and most favourable adsorption sites for propylene, propylidyne, and all of the C3Hx (x = 3-7) intermediates (1-propyl, 2-propyl, propylidene, 1-propenyl, 2-propenyl, propenylidene, and propynyl). All surface species are more stable than gas-phase propylene. Propylidyne is found to be the most stable species at the surface, in agreement with previous experiments. All surface moieties adsorb on sites where the metal atoms replace the missing hydrogen, thereby preserving a sp(3) hybridisation of the C atoms of the adsorbed hydrocarbon. We used the adsorption energies to compute the overall reaction energies for a total of 18 elementary C-H bond activation and isomerisation reactions that are likely to be involved in the dehydrogenation of propylene. The combination of energy and vibrational frequency calculations allowed us to propose some species as possible intermediates of the decomposition process-propylidene and I-propenyl. (c) 2006 Elsevier Inc. All rights reserved.