A modified form of the Thiele modulus was proposed for the reaction of n-heptane in the bidispersive grain of the platinum reforming catalyst. To assess the rate of conversion under conditions of diffusion use was made of a modified formula, which incorporates the bidispersivity of the grain. An algorithm was proposed for the assessment of grain efficiency and reaction rate related to the parameters of the porous catalyst grain structure. The contribution of the porous structure was analysed, and its optimisation was carried out by maximising the grain effictiveness factor and reaction rate under conditions of diffusion. (C) 2005 Elsevier Ltd. All rights reserved.