When a porous solid catalyst is added to a fluid reactant, the reaction process becomes immediately much more complex with respect to the homogeneous single-phase process. Indeed, since the reaction now takes place essentially at the surface of the porous solid, diffusional heat and mass transfer intrusions may intervene. Then the kinetic analysis of heterogeneous catalytic reactions cannot neglect the effect of such intrusions, since the rate of the reaction now corresponds to the slowest step of the overall catalytic process, the so-called rate-determining step, and the heat and mass transfer intrusions may affect considerably the relative rate of such steps.