The surface reactions involved in the palladium-catalyzed formation of benzene from acetylene are discussed as an example of the way in which such strategies can be used to understand catalytic reaction pathways in detail. This provides an ideal system for surface science/catalysis studies since benzene is formed both in ultrahigh vacuum in temperature-program med desorption and under high-pressure, catalytic conditions. It is found that benzene is synthesized on clean Pd(1 1 1)by an initial fast reaction between two adsorbed acetylene molecules to yield a tilted C4H4 metallacyclic intermediate. This reacts with a third acetylene to form benzene and the hexagonal (1 1 1) surface of palladium acts as a template for the reaction. Under catalytic conditions, however, it is found that a relatively unreactive vinylidene layer covers the surface. Organometallic studies show that acetylene and vinylidene can react to form a C4H4 intermediate analogous to that found to form from acetylene on a palladium surface. This suggests an alternative possible reaction pathway under catalytic conditions in which acetylene and vinylidene react to eventually form benzene. This proposal is confirmed using nuclear magnetic resonance analysis of a C-13-labelled vinylidene-covered surface. It is also shown that the vinylidene species formed from acetylene and ethylidyne species formed from ethylene can react with hydrogen at high pressures where the ethylidyne removal rate is found to be first-order in hydrogen pressure. This behavior is used to explain the hydrogen pressure dependence during catalysis where the hydrogen performs two roles, that of reactant, and to remove carbonaceous species from the surface. Finally, it is found that vinylidene also reacts with hydrogen at high pressures where it is removed via the formation of ethylidyne. (C) 2001 Elsevier Science B.V. All rights reserved.