Abstract A simple model for the catalytic oxidation of CO on fractal lattices is studied via Monte Carlo simulations. We observe a strong influence of the lattice structure on the system behaviour. In the case of extremely fast particle diffusion the structure of the lattice becomes unimportant. We are able to explain these effects qualitatively by the aspect of particle segregation. It is shown that this reaction model belongs to the universality class of the Reggeon field theory.