Anaerobic biodegradation of fermented spent sulphite liquor, SSL, which is produced during the manufacture of sulphite pulp, was investigated. SSL contains a high concentration of lignin products in addition to hemicellulose and has a very high COD load (173 g COD 1(-1)). Batch experiments with diluted SSL and pretreated SSL indicated a potential of 12-22 1 methane per hire SSL, which corresponds to 0.13-0.22 1 methane (g VS)(-1) and COD removal of up to 37%. COD removal in a mesophilic upflow anaerobic sludge blanket, UASB, reactor ranged from 10% to 31% at an organic loading rate, OLR, of 10-51 g (1 d)(-1) and hydraulic retention time from 3.7 to 1.5 days. The biogas productivity was 3 1 (1(reactor) d)(-1), with a yield of 0.05 1 gas (g VS)(-1). These results suggest that anaerobic digestion in UASB reactors may provide a new alternative for the treatment of SSL to other treatment strategies Such as incineration. Although the total COD reduction achieved is limited, bioenergy is produced and readily biodegradable matter is removed causing less load on post-treatment installations.