An advanced kinetic and diffusion model was developed and verified for the oxidation of starch by hydrogen peroxide in the presence of a homogeneous iron tetrasulphonatophtalocyanine (FePcS) catalyst. The model takes into account several experimentally confirmed observations, such as the oxidative formation of carbonyl and carboxyl groups in starch, as well as the decomposition of starch, catalyst and H2O2. The model is based on molecular mechanisms for the oxidation and decomposition reactions as well as on the dual structure of starch particles comprising an outer reaction layer and an internal porous layer, in which diffusion resistance retards the reaction rate. Adsorption of the catalyst on the starch surface is included in the model. The mathematical model explained very well the experimentally observed complex behavior of starch oxidation kinetics and hydrogen peroxide decomposition.