The purposes of this study were to prove that ammonia can be present in biogas from anaerobic digestion and to control this ammonia by reducing the reactor pH. Ammonia containing biogas was produced for a period of more than 100 days, with a maximum of 332 ppm. Especially during periods of high free ammonia concentrations in the reactor was ammonia present in biogas. The free ammonia was effectively reduced to less than the inhibition level by pH-based control and the ammonia in biogas concentration was reduced to 9 ppm. Simultaneously the CH4-yield was severely reduced. A pH-based control of ammonia in biogas with a satisfactory biogas production was thus so far proven not to be achievable. In the carrying out this study it was shown that high ammonia concentrations lead to a range of problems: process inhibition, decreased COD removal efficiency, reduced biogas production, malodour and a poor biogas quality that requires treatment. (c) 2005 Elsevier Ltd. All rights reserved.