The chemistry of ethylene on Pt(111) single-crystal surfaces was chosen here to represent olefin hydrogenation reactions on transition-metal catalysts. In vaccum the hydrogenation of ethylene was proven to proceed via a stepwise incorporation of hydrogen atoms on the clean surface, but under high pressures the catalyst was shown to become covered with carbonaceous deposits soon after exposure to the reactant gases. The species that compose the strongly bonded hydrocarbon fragments were identified as ethylidyne, a C-2 moiety where one carbon atom sits on a 3-fold hollow site on the surface and is single-bonded to a methyl group directly above it. In order to better understand the role of the ethylidyne layer in the hydrogenation reaction, the mechanism of the conversion of ethylene to ethylidyne was studied in some detail. Even though no simple scheme has been found to explain this surface process so far, our studies have led to the rejection of previously suggested two-step pathways involving either vinyl or ethylidene intermediates. Vinyl moieties were shown to undergo a series of reactions and to form a family of intermediates, including ethylene, before ultimately transforming into ethylidyne. The involvement of ethylidene in any simple two-step mechanism was shown to also be inconsistent with results from kinetic studies using trideuteroethylene. Finally, the participation of ethyl groups was discarded on the grounds that they decompose readily via a beta-hydride elimination step into ethylene. The participation of any of those intermediates in the mechanism for ethylene conversion could nevertheless be possible if they were to reach a fast pre-equilibrium with the chemisorbed ethylene and to then decompose slowly to ethylidyne. Unfortunately, further testing of this hypothesis is hindered by the added complications due to the non-first-order kinetics of the ethylidyne formation and the competition of that reaction with other H/D exchange and hydrogenation steps. It is also not clear yet what role these ethylidyne moieties may play in the mechanism of the high-pressure catalytic reactions, but several options are discussed here, including the possibility of them acting either as hydrogen transfer agents or simply as site blockers on the surface.