The performance and microbial community characteristics of a laboratory scale anaerobic baffled reactor (ABR) with four compartments (C1-C4) treating sugar refinery wastewater were investigated. The COD removal was 94.8 % with a CH4 yield of 0.21 L g(-1) CODremoved at total organic loading rate (OLR) of 5.33 kg COD/m(-3) day(-1). Fermentative bacteria were dominant in C1 and C2, while syntrophic acetogens and methanogens were dominant in C3 and C4. Some acid-tolerant methanogens were enriched in acidogenic phase. The present of the acid-tolerant methanogens could improve the efficiency and stability of the ABR as the most of the methanogens are vulnerable to low pH. In addition, high functional redundancy of the fermentative bacteria implicated that the microbial communities in acidogenic phase were stable functionally and allowed the ABR to balance perturbation. It was also found that syntrophic acetogenesis might be a weakness in the ABR as syntrophic acetogens were poor as compared with fermentative bacteria and methanogens.