A kinetic model of the reactions involved in the removal of aqueous aromatic compounds from wastewater by horseradish peroxidase (HRP, also known as hydrogen peroxide oxidoreductase, EC 1.11.1.7) has been calibrated and verified for phenol removal in the presence and absence of excess polyethylene glycol (PEG), a protective additive. Model expressions are used to describe inhibition of peroxidase through compound III formation and permanent inactivation through interactions of the enzyme with free radical products and through entrapment in polymer precipitates. In the absence of PEG, the model accurately predicts the time course of conversion of phenol and indicates that the majority of inactivation may be modelled as being due to free radicals. Application of the model also indicates that permanent enzyme inactivation in the presence of excess PEG may be modelled as being due to inactivation by end-product polymer alone. The rate constant associated with compound III decomposition in the presence of phenol was found to be approximately ten-times larger than the value reported in the literature for the decomposition of compound III in the absence of a reductant substrate.