The biodegradation and inhibition kinetics of a commercial nonylphenol ethoxylate formulation were modelled for an activated sludge system fed with a synthetic substrate and nonylphenol ethoxylate mixture. Kinetic and stoichiometric coefficients of the proposed activated sludge model were obtained by employing on-line respirometry. Experimental as well as model results confirmed that nonylphenol ethoxylate exhibited non-competitive inhibition on the hydrolysis process with a coefficient of 150 mg/L on the basis of COD and negatively influenced biomass growth through a competitive inhibition mechanism with a coefficient of 500 mg/L on the basis of COD, when the biomass was not acclimated. Upon acclimation of the activated sludge system, the inhibition concentration for non-competitive inhibition on hydrolysis was increased to 5000 mg/L, practically showing no inhibition, and the coefficient of competitive inhibition increased to 450 mg/L., corresponding to a significant decrease in the inhibitory effects of NPEO on growth. (c) 2010 Elsevier Ltd. All rights reserved.