When an inhibitory substrate, phenol, was treated under mesophilic conditions (25, 30, 35, and 40• C), the upflow anaerobic sludge bed (UASB) reactors at 30• C resulted in the greatest amount of biomass and the largest granule size, while the UASB reactors at 25• C resulted in the smallest granule size and the greatest amount of wash-out of sludge. The granule size tended to be negatively correlated with the amount of wash-out of sludge. With an increase in temperature, the kinetic constant k for anaerobic phenol degradation increased and the half saturation constant (K s ) decreased. The mass fraction of methanogens (f ) increased with increasing operational temperature in the UASB reactors and the activation energy (E a ) for acetate methanogenesis was larger than that for phenol acidogenesis in the batch reactors, indicating that the operational temperature imposes a more influential effect on methanogens than on acidogens. From the results of the activity of acidogens and methanogens (expressed in specific COD utilization rate), the rate-limiting step is phenol acidogenesis.