Effectiveness factor of a non-isothermal porous catalyst pellet with a bimodal pore size distribution is a strong function of the dimensionless parameter alpha, which is proportional to the ratio of diffusion resistances in the macro and micropores. For exothermic reactions, the maximum observed in the effectiveness factor versus particle-Thiele modulus curves shifts to lower particle-Thiele modulus values and also becomes more significant with an increase of alpha. In the prediction of observed rates, possible effects of four dimensionless groups, namely particle-Thiele modulus phi(i), alpha, Prater parameter beta and gamma should be considered for porous catalysts having bimodal pore-size distributions.