BACKGROUND: Mathematical models describing the physical, chemical and biological processes that take place in bioremediation are necessary to design and optimise these technologies. This work models the effect of toluene as a gaseous cosubstrate in the degradation of phenanthrene in soil, considering the consumption of pollutants, the production of intermediate degradation compounds and mineralisation. The proposed model consists of a set of sequential reactions to convert phenanthrene to carbon dioxide and biomass with the production and consumption of phthalic acid, which is the main intermediate metabolite. RESULTS: The considerations of the model were supported by experimental data, and it was evaluated for phenanthrene degradation kinetics with previously reported packed column reactor experiments. The mathematical model proposed describes the mineralisation of phenanthrene accurately and also predicts a reduced accumulation of phthalic acid when toluene is added as cosubstrate. The model fits the experimental data of phenanthrene degradation when toluene is added but slightly overestimates the residual phenanthrene in the control case. CONCLUSION: The simplified model of sequential reactions represents the column experiments (P < 0.05) for phenanthrene degradation and mineralisation with toluene as cosubstrate, considerating the production and consumption of phthalic acid. (C) 2008 Society of Chemical Industry