A multigrain catalyst model is developed to describe the simultaneous multicomponent diffusion-cum-reaction taking place in a unifunctional multicomponent catalyst. In this, particles of the more active component are randomly dispersed in a less active near-spherical porous matrix. The catalyst macro-particle is assumed to consist of sub-layers of different thicknesses of active matrix, M, alternating with sub-layers (of different thicknesses) of microspherical particles of the more active component (Z) embedded in the active matrix (the Z + M sub-layer). Cracking of a model compound, 1,3,5-triisopropylbenzene (1,3,5-TIPB), in this catalyst particle is selected as a simple test problem. The concentration profiles of the reactant and the products within the catalyst macro-particle are computed and the effects of important parameters on these are studied. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.