A mathematical model that describes the heterogeneous reaction-diffusion process involved in a batch reactor with immobilized enzyme is presented. The model is based on equations considering reaction and diffusion components including biocatalyst particle size distribution. The reaction system includes the bulk liquid phase containing the dissolved substrate (and products) and the solid biocatalyst phase represented by spherical porous particles carrying the enzyme. The model developed is illustrated for the case of penicillin G hydrolysis with immobilized penicillin acylase, which is a complex reaction system in which both products of reaction and the substrate itself are inhibitors. Significant differences in batch reactor performance simulation are observed when considering biocatalyst particles of a single radius and particle size distribution. The magnitude of these differences is proportional to the dispersion (standard deviation) considered in that size distribution function. (C) 2010 Elsevier BM. All rights reserved.