Coke formation rates under propane dehydrogenation reaction conditions on a used monolithic Pt/gamma -Al2O3 catalyst have been experimentally determined in a thermogravimetric analyser (TGA) as a function of time on stream covering wide temperature and concentration ranges. For relatively short times on stream, especially at low temperatures and low propylene concentrations, a remarkable initial quadratic increase has been observed in the coke formation rates versus time with a high apparent propylene reaction order. After longer times on stream the coke formation rate decreases to a constant residual coke growth above approximately 12 wt.% coke content. The experimental data have been successfully described by a kinetic rate expression based on a mechanistic dual coke growth model. In this model it has been assumed that initially coke precursor is formed via a propylene oligomerisation process, explaining the observed auto-catalysis for short times on stream.