The kinetics of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) could be differentiated during simple respirometric and titrimetric assays. The following properties were applied: NOB are more sensitive at low concentrations of oxygen than AOB, and it is only during the ammonium oxidation catalyzed by AOB that protons are released. For the AOB, the saturation coefficients by oxygen (K'(AOB)) and substrate (K-sAOB) were found to be 0.99 g O-2/m(3) and 0.3 g N-NH3/m(3) respectively. For the NOB, the saturation coefficients by oxygen (K'(NOB)) and substrate (K-sNOB) were found to be 1.4 g O-2/m(3) and 2.2 x 10(-4) g HNO2/m(3), respectively. The respirometric and titrimetric assay was a quick and reproducible method for determining kinetic coefficients of nitritation and nitratation separately. Furthermore, through titrimetry, the concentration of ammonia could be evaluated on-line by means of the consumption of a pH-neutralizing solution of Na2CO3 (0.2 M). (c) 2006 Elsevier Ltd. All rights reserved.