The paper deals with the thermal degradation of starch during phosphorylation carried out at elevated temperatures in the solid phase to produce biodegradable flocculants. The aim of the work reported here was to elucidate the mechanism and kinetics of the degradation process. In the first part of this study, a discrete Markov-chain model was proposed and discussed. The whole molecular weight range of the raw material and degradation products was divided into 10 discrete intervals. Mass transitions between them were considered as the consequence of splits of molecules to fragments, occurring with given probabilities at different positions within the molecules during a time step. Supposing various distributions of split probabilities along the length of starch molecules, and considering different influences of molecular weights on these probabilities, simulations were carried out resulting in characteristic patterns in the evolution of molecular weigh distributions during the process. These characteristics made it possible to identify the mechanism of degradation.