The influence of catalyst surface structure on a model hydrocarbon hydrogenation reaction is described. A Monte Carlo model for the hydrogenation of hydrocarbons by supported metal catalysts is presented and supplemented by experimental data for the hydrogenation of ethene by Pt/silica. It is proposed that the apparent structure sensitivity exhibited by this reaction can be attributed to the influence of particle size on the reaction kinetics. Specifically, we demonstrate that the location of the kinetic discontinuity separating two regimes of differing kinetic behaviour is dependent on the size of the Pt particles. We attribute thIS pparent structure sensitivity to the relative ease of forming a surface-reaction inhibiting layer of adsorbed hydrocarbon on the surface of small Pt particles. It is further shown that hydrogen spillover and diffusion do not influence the reaction kinetics to a significant extent.