Waste from the leather industry, known as limed leather fleshing (LF), has a low C:N (3.2) and ail alkaline pH of 11.4. This is a major disadvantage for anaerobic digestion due to ammonia toxicity for methanogenesis. This study describes co-digestion of LF with biodegradable fraction Of municipal solids waste Optimised over a range of C:N and pH to minimise ammonia and to maximise biogas yield. The optimum conditions were found with a blend that provided C:N of 15 and pH of 6.5 and the Cumulative biogas yield increased from 560 mL using LF fraction alone, to 6518 mL with optimum blend. At higher pH of 8.5. unionised ammonia was high (2473 mg L(-1)) coincided with poor biogas yield (47 mL d(-1)) that confirms ammonia toxicity. By contrast at a pH of 4.5 the ammonia was minimum (510 mg L(-1)), but high VFA (26,803 mg L(-1)) inhibited the methanogens. Biomass activity measured using ATP correlated well with biogas yield as reported previously. (C) 2009 Elsevier Ltd. All rights reserved.