The effect of pH and nitrite concentration on the activity of the nitrite oxidizing bacteria (NOB) in an activated sludge reactor has been determined by means of laboratory batch experiments based on respirometric techniques. The bacterial activity was measured at different pH and at different total nitrite concentrations (TNO(2)). The experimental results showed that the nitrite oxidation rate (NOR) depends on the TNO(2) concentration independently of the free nitrous acid (FNA) concentration, so FNA cannot be considered as the real substrate for NOB. NOB were strongly affected by low pH values (no activity was detected at pH 6.5) but no inhibition was observed at high pH values (activity was nearly the same for the pH range 7.5-9.95). A kinetic expression for nitrite oxidation process including switch functions to model the effect of TNO(2) concentration and pH inhibition is proposed. Substrate half saturation constant and pH inhibition constants have been obtained. (C) 2011 Elsevier Ltd. All rights reserved.