The reaction pathways for acetylene trimerization and hydrogenation, and ethylene hydrogenation, catalyzed by palladium, are explored using a range of surface-sensitive techniques. Reflection-absorption infrared spectroscopy (RAIRS) and low-energy electron diffraction (LEED) show that ethylene is di-sigma-bonded on clean Pd(111), but forms a pi-bonded species on a hydrogen pre-covered surface, where the transformation is induced by sub-surface hydrogen. Catalytic ethylene hydrogenation proceeds on an ethylidyne-covered Pd(111) surface. and it is found that ethylene can still adsorb onto palladium in spite of the presence of an ethylidyne overlayer, and is still in a di -sigma-con figuration. The rates of ethylidyne formation, and removal by hydrogen are measured independently, where the latter rate is found to be first order in hydrogen pressure. The ethylidyne coverage is measured under reaction conditions as a function of P(H-2)/P(C2H4) and found to decrease to -1/3 of saturation. Benzene is formed from acetylene on clean Pd(111) via a rnetallocyclic C4H4 intermediate. This further reacts with a third acetylene to form benzene. However, catalytic cyclotrimerization proceeds in the presence of a carbonaceous layer, which consists of vinylidene species (CH2=C=). Thus, at high pressures, benzene is formed by reaction between acetylene adsorbed on the vinylidene-covered palladium surface and adsorbed vinylidene itself. The addition of high pressures of hydrogen to the reaction mixture forms a more open surface covered by a mixture of ethylidyne and vinylidene species, rationalizing the observed increase in the benzene formation rate with the addition of hydrogen. (C) 2004 Elsevier B.V. All rights reserved.