Most of the kinetic studies on nitrification have been performed in diluted salts medium. In this work, the ammonia oxidation rate (AOR) was determined by respirometry at different ammonia (0.01 and 33.5 mgN-NH3 L-1), nitrite (0-450 mg N-NO2- L-1) and nitrate (0 and 275 mg N-NO3- L-1) concentrations in a saline medium at 30 degrees C and pH 7.5. Sodium azide was used to uncouple the ammonia and nitrite oxidation, so as to measure independently the AOR. It was determined that ammonia causes substrate inhibition and that nitrite and nitrate exhibit product inhibition upon the AOR. The effects of ammonia, nitrite and nitrate were represented by the Andrews equation (maximal ammonia oxidation rate, r(AOMAX), = 43.2 [mg N-NH3 (gVSS(AO)h)(-1)]; half saturation constant, K-SAO, = 0.11mg N-NH3 L-1; inhibition constant K-IAO, = 7.65 mg N-NH3 L-1), by the non-competitive inhibition model (inhibition constant, K-INI, 176 mg N-NO2 (-) L-1) and by the partially competitive inhibition model (inhibition constant, K-INA, 3.3 mg N-No-3(-) L-1; alpha factor = 0.24), respectively. The r(AOMAX) value is smaller, and the K-SAO value larger, than the values reported in diluted salts medium; the K-IAO value is comparable to those reported. Process simulations with the kinetic model in batch nitrifying reactors showed that the inhibitory effects of nitrite and nitrate are significant for initial ammonia concentrations larger than 100 mg N-NH4+ L-1. (c) 2005 Society of Chemical Industry.