Abstract. Temperature is a very important parameter during nitritation, having a direct effect on ammonia oxidation rate (AOR) and enzymatic activities which relate to both N 2 O and NO emission. This study aims at investigating the effect of temperature on AOR, N 2 O and NO production in an enriched ammonia oxidizing bacteria (AOB) sequencing batch reactor (SBR) performing partial nitrification (PN) of synthetic reject wastewater. To achieve that, a SBR was subject to several shifts in temperature (in the range of 30 to 15°C, 5°C for each decrease). Cycle studies, which contain two aeration phases, were conducted under each temperature. The results showed that AOR specific exponentially correlates with the temperature during the temperature decreasing experiments. With the decrease of the temperature, N 2 O firstly increased and then dropped to very low levels along with the decrease of the AOR, unlike NO that did not show any apparent connection with the temperature.