Treating a synthetic wastewater containing phenol as the sole substrate at 26 degrees C, an upflow anaerobic sludge blanket reactor was able to remove over 98% of phenol up to 1,260 mg/l in wastewater with 12 h of hydraulic retention time, corresponding to 6.0 g-COD/(l x day). Results showed that benzoate was the key intermediate of phenol degradation. Conversion of benzoate to methane was suppressed by the presence of phenol. Based on DNA cloning analysis, the sludge was composed of five groups of microorganisms. Desulfotomaculum and Clostridium were likely responsible for the conversion of phenol to benzoate, which was further degraded by Syntrophus to acetate and H2/CO2. Methanogens lastly converted acetate and H2/CO2 to methane. The role of epsilon-Proteobacteria was, however, unclear.