The aerobic ammonia oxidation is an important process for nitrogen removal from wastewater with high ammonia concentration. Here, we investigated the metabolic properties of a mixed culture of aerobic ammonia oxidizers and determined its optimal reaction conditions. The maximum specific rate of NO2--N formation of the culture was 14.9 mg N mg (protein)(-1) d(-1). The ammonia oxidation capacity was positively correlated with the microbial biomass concentration. Haldane model showed that the half-saturation constant (K-s) for NH4+-N of the culture was 78.5 mg L-1, and the inhibition constant (K-i) for NW4+-N was 393.4 mg L-1. The acetic acid and beef extract had significant inhibitory effects on ammonia oxidation, while there was little effect of methanol and glucose on ammonia oxidation. Orthogonal experiments with range analysis, variance analysis, factor contribution analysis and response surface analysis confirmed the optimal reaction conditions for ammonia oxidation to be 30-32 degrees C, pH 7.8-8.0, and 150-200 mg L-1 NH4+-N concentration. (C) 2011 Elsevier Ltd. All rights reserved.