More or less severe nitrification inhibition was observed in several pilot and full-scale activated sludge plants treating industrial wastewaters. In order to control the treatment process under inhibiting conditions, extended nitrification models have been developed on base of the ‘Activated sludge model No. 1’. In the case of temperatures between 25 and 30°C, the nitrification process has been expressed as a two-step reaction with nitrite as intermediate. Model elements for competitive and non-competitive inhibition as well as for biodegradation of the inhibitor were added, if required. The dynamic behaviour of the investigated activated sludge systems indicates that there are biodegradable non-competitive inhibitors. Operational as well as simulation results show that nitrifying activated sludge plants may become acclimatized to inhibitory compounds but have to be protected from peak loads of both nitrogen and inhibitory compounds.