A mathematical model which describes the effect of pH and dissolved O2 on growth and activity of Nitrosomonas and Nitrobacter in mixed culture is presented. A good fit is obtained between experimental data and theoretical curves computed with parameters estimated by independent methods. Experimental data together with simulation studies show that inadequate aeration induces a temporal shift of ammonium and nitrite oxidations resulting in a transient nitrite accumulation similar to that caused by an increase of temperature.