The nitrification process (ie biological ammonium oxidation to nitrate) is a two-step process with nitrite as an intermediate product. As it is an aerobic process, its kinetics is highly dependent on the dissolved oxygen (DO) concentration in the medium. However, the influence of this limitation on the nitritation (first step) is shown to be less important than in the nitratation (second step). This dependence on DO concentration is generally described using a Monod-type kinetics with K-o as the oxygen affinity constant. In this work, a procedure for the calculation of both affinity constants is presented. This procedure is based on monitoring the DO drop in the reactor when external aeration is stopped and the biomass is consuming without substrate (ammonium or nitrite) limitations. This methodology includes the contemplation of the oxygen transfer from the atmosphere, the response time of the DO probe and the inhibition of the nitratation step with sodium azide when estimating K-OA (nitritation oxygen affinity constant). The results obtained are K-OA = 0.74 +/- 0.02 mg O-2 dm(-3) and K-ON = 1.75 +/-0-01 Mg O-2 dm(-3). Moreover the influence of the aforementioned considerations on the estimated K-o values is also discussed. (c) 2005 Society of Chemical Industry.