The feasibility of thermophilic (55 degrees C) anaerobic sulphur removal from partly acidified wastewater was investigated using two 6.5-1 upflow anaerobic sludge bed (UASB) reactors (R1 and R2). Both reactors were inoculated with a mixture of mesophilic sulphidogenic sludge and thermophilic methanogenic sludge (ratio 1:1) and were fed with a sucrose:propionate:butyrate mixture in a chemical oxygen demand (COD) ratio of 2.1.1. Initially, reactor R1 was supplied with this feed supplemented with a high sulphate concentration (COD/SO42- ratio of 1.33), while the feed of reactor R2 contained low sulphate levels (COD/SO42- ratio of 6.67). The reactors were operated at a hydraulic retention time of 3.7 h and the imposed volumetric organic loading rates ranged from 4.9 to 19.8, and from 4.9 to 46.5 g COD 1(-1) day(-1) for R1 and R2, respectively. A complete acidification of sucrose occurred in both R1 and R2. The extent of sulphate reduction depended on the imposed COD/SO42- ratios. R1, when operating at an organic loading rate (OLR) and sulphate loading rate (SLR) of 19.8 gl(-1) day(-1) and 14.8 gl(-1) day(-1), respectively, achieved a maximum sulphate reduction efficiency of 50%. In the case of a COD/SO42- ratio of 6.67 (R2), sulphate reduction efficiencies exceeding 95% were achieved at an OLR and SLR of 46.5 g l(-1) day(-1) and 7.0 g l(-1) day(-1), respectively. In both reactors, the effluent sulphide concentrations were always below 400 mg l(-1), of which similar to 90% was present as undissociated H2S, (under the given conditions similar to pH 5.8-6.1 and 55 degrees C). The incomplete sulphate reduction in R1 could be attributed to the limited availability of required reducing equivalents. The biogas (including CH4 and CO2) production rates in R1 were very low, i.e. 0.5 1 biogas l(-1) reactor day(-1), resulting in negligible amounts (<10%) of H2S stripped from the reactor liquid. Introduction of N-2, as an additional strip-gas (flow rate - 20 1 l(-1) day(-1)) into R1 resulted in an almost complete H2S removal. In R2, the biogas production rates reached similar to 3 1 l(-1) day(-1) at an OLR of 38.5 g l(-1) day (-1). This resulted in a H2S stripping efficiency of similar to 50%. (C) 2000 Elsevier Science Ltd. All rights reserved.