About 0.4-0.5 Tg nitrogen (1 Tg = 10(9) kg) are annually channeled through German waste water treatment plants. To quantify the gaseous N2O losses during the conversion of the above nitrogen inputs in situ measurements were carried out in the activated sludge, the nitrification-denitrification unit and the outflow of the waste water treatment plant of Giessen, Germany, during 1993-1996. Self-constructed floating PVC chambers were used to collect N2O. The gas was sucked and trapped on molecular sieve traps and quantified in the laboratory by gaschromatography (ECD). Until 1994 the municipal waste water of Giessen was purified only by an activated sludge unit. From 1994-1996 the waste water of meanwhile 150.000 inhabitants was splitted and purified partly by the activated sludge and partly by newly installed nitrification-denitrification units. Since 1996 the waste water is purified completely by nitrification-denitrification. In addition to N2O emissions, NH4+-N, NO3--N, NO2--N, BOD5, water temperature, pO(2) pH and the N2O dissolved in the waste water were determined. The dissolved N2O was quantified gaschromatographically after heating the samples for 80 min at 80 degrees C.