Recent experimental studies of alkene and alkyne hydrogenation reactions have shown the presence of a transition between a reactive regime at low hydrocarbon surface coverage and a less reactive regime in which the catalyst surface is saturated with the hydrocarbon. This transition is discontinuous but reversible. Langmuir-Hinshelwood rate expressions cannot account for this discontinuity between the two regimes, indicating that the origin of the transition is related to the non-random distribution of reactants on the catalyst surface. Time-dependent Monte Carlo simulations of ethylene hydrogenation have been used to predict the presence of this transition and investigate the complex dynamics of the reaction.