A new spherical random network model is applied to the problem of diffusion and reaction occurring simultaneously in porous catalysts. The diffusion/reaction equations are solved numerically over the network for the case of a single

isothermal

irreversible first - order reaction. The simulation results show that calculated tortuosities varies with network connectivity as well as the Thiele modulus. This work also suggests that tortuosities obtained under diffusion - only conditions can only be used under the strong reaction control conditions. The simulation results obtained from different network models are carefully analysed and the variation of those are theoretically discussed.