Iron salts are often used in activated sludge treatment plants as coagulants or to improve reactor performance. Previous studies have indicated that iron itself has an impact on the activated sludge process. However, the interaction of iron with nitrite or nitrate present in the sludge has received little attention. In this research, the influence of addition of Fe(II) or Fe(III), alone or together with NO2 or NO3 on bench-scale activated sludge reactors was examined. Large differences were established between the distinct treatments, regarding reactor performance, sludge characteristics as well as microbial community. Ferric iron was more detrimental than ferrous iron. In some cases, nitrite was found to enhance inhibitory effects of the added iron, whereas nitrate had more a neutralizing effect. It was found that precipitation of phosphate by the iron was not responsible for the observed inhibition. Decrease in pH upon formation of iron hydroxides and the impairment of the floe structure could partially explain the toxicity of the iron dosages. The formation of toxic nitrogen oxides, such as nitric oxide, can also be of importance. The observed positive effect of nitrate on the floe activity is of interest and warrants further elucidation. (C) 2003 Elsevier Science Ltd. All rights reserved.