The results of a series of dynamic Monte Carlo simulations of the hydrogenation of acetylene and of the selective hydrogenation of acetylene from acetylene/ethylene mixtures in the presence of trace quantities of carbon monoxide are presented. We demonstrate that, as with previous simulations of ethane hydrogenation, both the hydrogenation of acetylene and of acetylene/ethylene mixtures exhibit kinetic discontinuities separating two regimes of distinct kinetic behaviour. For the hydrogenation of acetylene/ethylene mixtures activation energies of 16.7 and 56.0 kJ mol(-1) are obtained for each of the two kinetic regimes, respectively. The possible role of trace quantities of carbon monoxide in enhancing the selectivity of the reaction is discussed with reference to the formation of isolated adsorption sites on the catalyst surface. We suggest that the enhanced selectivity toward the hydrogenation of acetylene in the presence of CO can be partly attributed to the formation of geometrically distinct adsorption sites on the catalyst surface which arise as a consequence of the random sequential adsorption of immobile hydrocarbon species on the catalyst surface. (C) 2004 Elsevier Ltd. All rights reserved.