A new approach is presented that combines kinetic and mechanistic considerations which take into account chemical reactions and catalyst deactivation in the modeling of the catalytic degradation of polymers. Though acid-catalyzed hydrocarbon cracking reactions involve a large number of compounds, reactions and catalyst deactivation and are very complex, the model gives a good representation of experimental results from the degradation of polyethylene and polypropylene over fluidized acidic catalysts. This model provides the benefits of product selectivity for the chemical composition such as alkanes, alkenes, aromatics, and coke in relation to the effect of structurally different polymer feeds, the performance of the catalyst and the selection of their particle sizes, and the influence of different reaction temperatures. It is an improvement of the currently available empirical "lumping" techniques which usually are severely restricted in terms of product definitions. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2591-2599, 2009