A stochastic model has been developed to account for the catalyst decay by coking based on the interaction of the geometries of the coke deposit and the pore structure. Coke is assumed to deposit randomly on the substrate macropores as well as in zeolite micropores resulting in both the active site coverage and pore blockage. The substrate pore structure is represented by a corrugated parallel bundle pore model. ?he mass balance equations are solved analytically to simulate conversion, coke content and surface area as a function of time-on-stream. Three different fouling mechanisms : parallel, series and triangular, are also investigated and from the simulated results, the series fouling mechanism is found to fit the experimental observations in a modified micro activity testing (MMAT) unit better.