In the last decade, the treatment of high ammoniacal concentration effluents has become of great interest. Various effluents can contain some hundred milligrams of nitrogen per litre (supernatants from anaerobic digestion, leachates from municipal landfill, ...) may need specific treatment before recycling them to the plant inlet. Partial nitrification process, leading to nitrites accumulation, is proposed here in a lab-scale sequencing batch reactor (SBR) at 30 degrees C. This reaction is carried out by maintaining strong ammoniacal concentrations (sequencing feed) which have the role of inhibiting the nitrite-oxidizing population responsible for the oxidation of nitrites into nitrates (final stage of nitrification). During the first experiments, nitrites accumulation has been obtained quite rapidly but the presence of nitrates has been noticed after 30-40 days. Consequently, the process had to be controlled and optimized. The mode of controlling the process, mainly based on the on-line determination of the oxygen uptake rate (OUR), indicator of the bacterial activity, leads to a maximal removed nitrogen load of 2 kg N m(-3) d(-1), with 100% of conversion of ammonium into nitrite during 190 days. A simplified mathematical model is proposed to describe the biological reactions taking place in the process, in order to determine the optimum conditions of experiment to obtain a stable nitrite accumulation without nitrate production. This model represents several features, such as the inhibition of ammonia-oxidizing bacteria by its substrate NH3 and product HNO2, and the inhibition of nitrite-oxidizing bacteria by free ammonia (NH3). It appears that the model correctly describes this nitritation process in sequencing batch reactor. (c) 2007 Elsevier B.V. All rights reserved.