For an integrated nitrification-denitrification process, nitrite formation in the aerobic stage leads to big savings. Recently experimental observations (Garrido et al., 1996) have shown that it is possible to obtain full ammonium conversion with approximately 50 % nitrate and 50 % nitrite in the effluent of a biofilm air-lift suspension reactor. With oxygen concentrations between 1 and 2 mg/l a maximum nitrite accumulation of 50 % was reached. Here we give a simple diffusion-reaction model describing these results. All the kinetic and mass transfer parameters were taken from the literature, except the mass transfer coefficient around the biofilm surface, which was fitted. The proposed model describes very well the measured data, despite the assumptions made. Using this model the influence of operational parameters was evaluated in order to establish ways to affect the NO2 concentration. None of these (surface loading, k1, pH) had a significant effect on the maximal NO2 accumulation. Controlling the oxygen concentration seems to be the most practical method to obtain optimal nitrification to nitrite in BAS reactors.