A bell-shaped empirical model was proposed to describe the pH-dependent behavior of the maximum specific substrate utilization rates (MSSUR, (q) over cap pH) of ammonium and nitrite oxidations. The model was developed using a cosine function. The model satisfactorily captured the bell-shape decrease of the MSSUR as the pH deviated from the optima. Analysis of 7 experimental data sets gave that the optimum pH (pH(opt)) lied in the range of 8.2 +/- 0.3 for ammonium oxidizing bacteria (AOB) and 7.9 +/- 0.4 for nitrite oxidizing bacteria (NOB). The pH range (w) within which the MSSUR maintained above a half of the greatest MSSUR (q) over cap (max)) was 3.1 +/- 0.4 for ACIB and 2.2 +/- 0.4 for NOB, which means that NOB is more sensitive to pH changes. Since the pH(opt) and the pH range (w) for NOB were smaller than those for AOB, it was inferred that a selective enrichment of AOB and consequent nitrite accumulation could be possible at an alkaline pH near or greater than the pH(opt) of AOB. (c) 2007 Elsevier Ltd. All rights reserved.