The capacity of nitrifying biomass, grown in biofilms or in suspension, to reduce NO2 - and NO3 - under anoxic conditions was tested in batch experiments. The estimated reduction rates were 5 and 25 mg N per gram volatile suspended solids (VSS) per day for nitrate and nitrite, respectively, in the case of the nitrifying biofilms. Activity tests carried out with successive feedings indicated that no acclimation of the biomass to the tested conditions occurred, as the obtained reduction rates remained almost constant. Another series of activity assays was carried out with nitrifying suspended biomass, and the reduction rates for nitrate and nitrite were 30.4 and 48.9 mg N per gram VSS per day, respectively. N2O and N2 were the final gaseous products, and their percentages depended on the source of nitrogen feed. The specific production of nitrous oxide during nitrification was investigated during continuous experiments in a biofilm airlift suspension reactor. Specific production rates up to 46 mg N 2O-N per gram VSS per day were measured. The percentage of N 2O produced represented up to 34.4% of the ammonia oxidized. Nitrite accumulation, low dissolved oxygen concentrations, and the presence of organic matter favored the production of nitrous oxide. N2O gas was not detected during the oxidation of nitrite even when organic matter was present. To prevent N2O gas production in nitrifying systems, the operation at low dissolved oxygen concentrations, nitrite presence, or organic matter content should be avoided.