The Fluid Catalytic Cracking Unit (FCCU) is a complex but important refinery process to catalytically crack heavy feedstock to lighter and low sulphur products. However, catalysts are subject to deactivation by coking, metal deposition and sulphur poisoning. Any improvement in either of the process or catalyst will enhance process benefits. To get more insights into the zeolite catalyst decay, theoretical and practical aspects of coking of a commercial zeolite catalyst have been investigated. Deactivation study was carried out in a laboratory scale fluidized bed reactor to disproportionate cumene as the model reaction. The material balance over the reactor system for the cumene can be precisely determined within 4%. Two theoretical catalyst pore structured models -’corrugated parallel bundle’ and ’stochastic network’ have been proposed to account for the catalyst coke deposition based on the interaction of the geometries of the coke deposit and pore structures. The prediction in coke content as a function of process time from the latter model agree well with the observed deactivation behaviour in the commercial catalyst. A number of structured catalyst parameters, which are not measurable by the existing analytical means can be estimated reliably from the models. Moreover, the coke deposition in the pore structures can be visualized.