The effect of mixing catalyst and inert particles on conversion has been studied. It has been shown that the dilution criterion proposed by van den Bleek et al. is not valid for all reaction orders. A new model (random particle model) which stochastically models a reactor consisting of a random mixture of catalyst and inert particles is proposed. Based on this model, dilution equations have been developed to enable an experimenter estimate a priori the magnitude of the effect of dilution.