In coordination with the EU-guidelines the large wastewater treatment plants in Switzerland have to be extended with enhanced nitrogen removal. For municipal wastewater treatment denitrification supported by an external carbon source instead of extending the plant may be an interesting and cost effective solution. At the wastewater treatment plant Zurich-Werdhölzli different experiments were performed with methanol addition to predenitrification from March to July 1994. Two parallel denitrifying lanes were investigated, one served for methanol addition experiments and the other as a reference. The adaptation period for methanol degradation was only a few days and nitrogen removal efficiency could be increased by 20%. The yield coefficient YCOD was 0.4 g CODX g-1 CODMe. Due to the low denitrification capacity nitrate peak loads cannot be fully denitrified. Hence, methanol as a carbon source requires more or less constant dosing.