The influence of crude glycerin on biogas production and methane content of the produced biogas was studied, when added to cattle slurry. The experimental design consisted of 5% wt (Gli 5), 10% wt (Gli 10), and 15% wt (Gli 15) of crude glycerin added to cattle slurry, and one control digester without addition of crude glycerin. Anaerobic digestion was carried out in 4 laboratory size CSTR-type biogas digesters with a working volume of 3 L, in semi-continuous regime at mesophilic conditions, over a period of 10 weeks. The highest biogas yields (825.3 mL g(-1) and 825.7 mL g(-1), respectively) relative to mass of volatile compounds added, were produced by the treatments Gli 5 and Gli 10. The control treatment produced 268.6 mL g(-1), whereas the treatment Gli 15 produced 387.9 mL g(-1). This low value was due to the breakdown of the process. Compared to the control, methane contents was increased by 9.5%, 14.3%, and 14.6%, respectively, for the treatments Gli 5, Gli 10, and Gli 15. (C) 2010 Elsevier Ltd. All rights reserved.