Aims: To study the effect of pH, temperature and substrate on the magnitude of N2O and NO production by heterotrophic nitrifiers. Methods and Results: The change in N2O and NO production by the heterotrophic nitrifiers Alcaligenes faecalis subsp. parafaecalis and Paracoccus pantotrophus because of variations in pH, temperature and substrate was studied in chemostat cultures under steady-state conditions. N2O, NO and CO2 production increased with temperature between 4 and 32 degrees C. For N2O an optimum temperature of 28 degrees C was observed. No optimum temperature was found for NO. Highest N2O and CO2 productions were observed at a pH of 7.0. However, besides having an optimum at pH 7.0, especially NO production but also N2O production increased significantly at pH <= 4.0. This increase in NO production under acidic conditions was partly because of chemo-denitrification, which contributed up to 62% of total NO production at pH 3.0 (0.8% for N2O). Furthermore, we could demonstrate that substrate quality significantly affects N2O, NO and CO2 production. N2O and especially NO production by A. faecalis p. was significantly lower on an ammonium citrate medium when compared with rates obtained for a peptone-meat extract medium. Conclusions: The results indicate that heterotrophic nitrifiers are suitable model organisms to study the influence of environmental factors on microbial N trace gas production. Significance and Impact of the Study: The results allow an improved description, e.g. of the pH dependency of N trace gas production by microbes and/or chemo-denitrification in process-oriented models describing the exchange of N trace gases between soils and the atmosphere.