The rates of biodegradation of acetic, benzoic, isophthalic, toluic and terephthalic acids were investigated in continuous stirred tank reactors employing a mixed bacterial culture. This culture was earlier adapted to additional substrates such as benzaldehyde, p-xylene, 1,4-dioxane, ethylene glycol, methanol and phenol, many of which are typically present in the effluents of PTA (purified terephthalic acid) and PFY (polyester filament yarn) plants. Damped oscillations indicative of predation were evident in the mixed culture. Direct microscopic observations confirmed the rise and fall of different microbial species during the course of biodegradation. Biological oxidation followed Monod kinetics, and the kinetic constants indicated that p-toluic acid was generally more difficult to degrade. With mixed substrates, the degradation of benzoic, terephthalic and p-toluic acids did not begin until most of the simpler compounds such as acetic acid and ethylene glycol were consumed; this observation suggests the applicability of cybernetic modelling to mixed culture systems. However, such a cybernetic model may not be expected to display a multiauxic growth curve though multiple substrates will be present. Recycling of biomass was not required in the present case. The mixed culture was unable to mineralize 1,4-dioxane alone; co;metabolism with acetic acid was observed.