The dependence of catalyst deactivation on cluster size is considered for poisoning and coking. A quantitative treatment of the size dependent deactivation was carried out for a heterogeneous catalytic mechanism with two kinetically significant steps. The corresponding kinetic equations are derived and numerically analyzed. A comparison with experimental data on methylcyclopropane hydrogenolysis over Pt with CO poisoning and ethylene hydrogenation on Pt with sulphur poisoning is given. For the latter case where there was more profound deactivation for smaller clusters, an adequate description of the final activity was achieved.