The effect of adding gallic acid, an inhibitory compound, and mixtures of gallic acid and glucose to well mixed batch digesters and to a two-stage digestion system was examined. The results of the batch studies showed that glucose (10 and 20 mg l(-1)) increased both the degradation of the gallic acid and the rate at which this occurred. They also showed that gallic add altered the amounts of ten hydrolytic enzymes in the sludge. The addition of glucose reversed this effect. The two-phase digester was operated with a hydraulic retention time of 3 hours in the acidogenic stage and 10.5 hours in the methanogenic reactor. The results showed that there was little removal of gallic acid in the acidogenic reactor. At concentrations of 10 and 20mg l(-1), gallic acid had little effect on the methane production. However, at a concentration of 50 mg l(-1) there was a drop of about 15% in the volume of methane produced. The addition of glucose (15 mg l(-1)) reversed this effect. A similar set of results was obtained for the removal of soluble GOD. An effect of adding gallic acid was to increase the concentrations of butyrate and propionate. It is not yet known how glucose operates to counteract these inhibitory effects.