A single particle model for the gas phase propylene polymerization including convection due to pressure drop in the particle is used to study the mass transfer of two component system. The addition of hydrogen in,small amounts increases the bulk diffusion of the propylene-hydrogen gas mixture, increasing the reaction rates (physical effects). As hydrogen also reactivates the active sites and increases the deactivation, these chemical effects along with mass' trans, fer effects are studied with empirical correlations. The experimentally observed increase in reaction rates in the presence of hydrogen may be due to physical and chemical effects combined together. The time scales for reaction and mass transfer of reactant in propylene-nitrogen mixture is analysed using the viscous modulus (a modified Thiele modulus) which accounts for effects of gas composition on mass transfer in porous particle due to convective flow and enrichment of the non-reacting inert. These time scales are also analysed for catalysts which show activation behaviour.