Emission of NO and N 2O from a full-scale two-reactor nitritation–anammox process was determined during a measurement campaign at the Dokhaven–Sluisjesdijk municipal WWTP (Rotterdam, NL). The NO and N 2O levels in the off-gas responded to the aeration cycles and the aeration rate of the nitritation reactor, and to the nitrite and dissolved oxygen concentration. Due to the strong fluctuations in the NO and N 2O levels in both the nitritation and the anammox reactor, only time-dependent measurements could yield a reliable estimate of the overall NO and N 2O emissions. The NO emission from the nitritation reactor was 0.2% of the nitrogen load and the N 2O emission was 1.7%. The NO emission from the anammox reactor was determined to be 0.003% of the nitrogen load and the N 2O emission was 0.6%. Emission of NO 2 could not be detected from the nitritation–anammox system. Denitrification by ammonia-oxidizing bacteria was considered to be the most probable cause of NO and N 2O emission from the nitritation reactor. Since anammox bacteria have not been shown to produce N 2O under physiological conditions, it is also suspected that ammonia-oxidizing bacteria contribute most to N 2O production in the anammox reactor. The source of NO production in the anammox reactor can be either anammox bacteria or denitrification by heterotrophs or ammonia-oxidizing bacteria. Based on the results and previous work, it seems that a low dissolved oxygen or a high nitrite concentration are the most likely cause of elevated NO and N 2O emission by ammonia-oxidizing bacteria. The emission was compared with measurements at other reject water technologies and with the main line of the Dokhaven–Sluisjesdijk WWTP. The N 2O emission levels in the reject water treatment seem to be in the same range as for the main stream of activated sludge processes. Preliminary measurements of the N 2O emission from a one-reactor nitritation–anammox system indicate that the emission is lower than in two-reactor systems.