Phenol biodegradation by an acclimated activated sludge was investigated in batch cultures with variable initial conditions of substrate and biomass (0.10 <= S-0/X-0 <= 1 g phenol g TSS-1). As conventional Haldane model could not explain the biomass growth observed after phenol exhaustion, a model based on the production and later consumption of an inhibitory metabolic intermediate was developed to describe the phenol biodegradation and the biomass growth profiles. This two-step model considers that biodegradation is accomplished by two microbial populations constituting the whole biomass. The new model depicts successfully both biomass and phenol courses by using a single set of kinetic parameters, and support the notion that the production of metabolic intermediates has a determinant role in phenol biodegradation kinetics. (c) 2005 Elsevier B.V. All rights reserved.