Both steps of nitrification (nitrite production and nitrite consumption) in the nitrifying reactor are differently affected by environmental conditions (oxygen concentration, pH-value, temperature, concentration of substrate and product) and operating conditions (sludge age) respectively. The understanding of these processes and the quantification of these effects allows to forecast the concentrations of unwanted toxic intermediates (nitrite) during nitrification as well as in the effluent of the waste water treatment plant (WWTP). Further it is possible to avoid or minimize high concentrations by operational procedures. The decisive parameters of nitrification in municipal waste water treatment were investigated in a pilot plant. From these experiments a mathematical model was derived and verified with data from a full scale WWTP.