Abstract
The time evolution of the reactant concentrations, the rate of production and the concentration of vacancies around islands of the minority species, in a model for the catalytic oxidation of carbon monoxide, is studied on percolation clusters by means of Monte Carlo simulations and scaling arguments. The model exhibits two kinetic transitions and the relevant exponents for the time evolution of the above-mentioned quantities are evaluated at criticality. The influence of the fractal substratum on the behaviour of the reaction is discussed and compared with reported results for homogeneous media.