Acid-catalysed hydrocarbon cracking reactions involve a large number of compounds, reactions and catalyst deactivation and are very complex. A new approach is presented that combines kinetic and mechanistic considerations which take into account chemical reactions and catalyst deactivation in the modelling of the catalytic degradation of polymers. The model gives a good representation of experimental results from the degradation of HDPE and PP over fluidised 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, and the performance of the catalyst used. It is an improvement of the currently available empirical "lumping" techniques which usually are severely restricted in terms of product definitions.