Simultaneous nitrification–denitrification (SND) allows biological nitrogen removal in a single reactor without separation of the two processes in time or space but requires adapted control strategies (anoxic/aerobic conditions). In this study, the formation of gaseous nitric oxide (NO G ) and nitrogen dioxide (NO 2G ) was monitored for SND in relation to the oxidation-reduction potential (ORP) and nitrogen removal in a lab batch reactor and a pilot membrane bio-reactor (MBR). In addition hospital wastewater (COD/N tot >6:1) was treated on site for 1 year. The highest total nitrogen removal rates of max 90% were reached at 220–240mV ORP (given as E h ) with corresponding maximal NO G emissions rates of 0.9μgg −1 VSSh −1 . The maximal emission rates of NO 2G (0.2μgg −1 VSSh −1 ) were reached at the same ORP level and the NO 2G emissions correlated to the nitrite accumulation in the activated sludge up to 5mgl −1 NO 2L -N. It was shown that this correlation was due to biological production and not due to pH-dependent chemical conversion. Therefore, NO 2G can be used as additional control loop for ORP-controlled SND systems to avoid the inhibition of denitrification and high nitrite concentrations in the plant effluent.