Abstract
Finite-size effects in kinetic (irreversible) phase transitions, from reactive to poisoned states, occurring in model reactions are interpreted with the aid of a phenomenological scaling approach. The proposed arguments are tested by computer simulations of a model for the oxidation of carbon monoxide on a fractal surface. The critical exponents of the transitions displayed by the model and the exponents for the transient period of the reaction at criticality are evaluated. A crossover from a reactive steady state to a regime where the surface could be poisoned by each of the reactants is found and discussed.