Density functional theory (DFT) calculations for acetylene hydrogenation on clean, vinylidene CCH2-covered (0.25 ML) and ethylidyne CCH3-covered (0.25 ML) Pt(111) surfaces were performed to probe the reaction mechanism and evaluate energetic changes due to high hydrocarbon coverage. A comparison between the reaction energetics on the clean and pre-covered surfaces shows that high coverage trends are similar for vinylidene and ethylidyne species: surface hydrocarbon species and hydrogen are destabilized by up to 150 and 30 kJ/mol, respectively. Unsaturated, multiply-bonded species are destabilized more than species forming fewer bonds with the surface. Activation energies are not affected, unless the spatial formation of a transition state is hindered or a reactant is significantly distorted. In these cases, activation barriers can be different by up to 50 kJ/mol and the relative significance of parallel steps may change. For example, CH2CH2 formation is hindered at high coverage and the relative propensity of CHCH2 for forming either CH2CH2 or CHCH3 is reversed. The calculations confirm that vinylidene CCH2 and ethylidyne CCH3 are spectator species in the overall reactions of ethylene and ethane formation. However, at the evaluated surface coverage of 0.25 ML, these spectator species may undergo hydrogen disproportionation with other hydrocarbon fragments, serving as a hydrogen reservoir and providing lower-energy pathways. As a result, the predicted energetics for acetylene hydrogenation at high coverage are affected by not only the extent of destabilization of active species and their transition states, but also by the relative stability of spectator species and their possible participation in disproportionation reactions. (C) 2004 Elsevier B.V. All rights reserved.