The advantages of enzyme process are numerous and well established. However, enzymes are mostly used as soluble catalysts, being poorly stable and hard to recover. These problems may be solved by the use of immobilised enzymes, but new problems may arise, especially mass transfer limitations. Internal mass transfer limitations that may be especially significant in microporous and gel matrices, which, on the other hand, are superior in terms of protein loading capacity. This paper deals with the study, using mathematical simulation, of the effect of reaction kinetics and particle size on the effectiveness factor of immobilised enzymes, subjected to internal diffusional restrictions. The main conclusion of this work is that, depending on the enzyme reaction mechanism, a particle size may exist at which the effectiveness factor is a maximum and, therefore, the immobilised enzyme behaviour is optimum.