This study models the biodegradation kinetics of two toxic xenobiotic compounds in enriched mixed cultures: a commonly applied herbicide (3,4-dichloropropionanilide or propanil) and its metabolite (3,4-dichloroaniline or DCA). The dependence of the metabolite degradation kinetics on the presence of the parent compound was investigated, as well as the influence of the feeding operation strategy. Model equations were proposed incorporating substrate inhibition of the parent compound and the metabolite during dump feed operation of a sequencing batch reactor (SBR). The kinetic parameters of the biomass were compared to step feed degradation of the SBR. The relationship between propanil and DCA degradation rates with the concentration of each compound was studied. A statistical comparison was carried out between the model predictions and experimental results. Substrate inhibition by both propanil and DCA was prominent during dump feed operation but insignificant during step feed. With both feeding strategies, the metabolite degradation was found to be dependent on the concentration of both the parent compound and the metabolite, suggesting that the DCA degrading enzymatic activity was independent of the detachment of the propionate moiety from the propanil molecule. After incorporating this finding into the model equations, the model was able to describe well the propanil and DCA degradation profiles, with an r (2) correlation >0.95 for each case. A kinetic model was developed for the degradation of the herbicide propanil and its metabolite DCA. An exponential inhibition term was incorporated to describe the substrate inhibition during dump feeding. The kinetics of metabolite degradation was dependent of the sum of the concentrations of metabolite and parent compound, which could also be of relevance to future xenobiotic modelling applications from wastewater.