An automatically controlled, glucose-fed, anaerobic digester was deliberately inhibited by addition of phenol. To overcome the phenol inhibition the feed dilution rate was lowered in such a way that the methane yield from glucose was kept the same as that under normal conditions. The concentrations of acetic and butyric acids remained below 100 mg/l, however, propionic acid accumulated to 2750 mg/l. Phenol apparently inhibited all tropic groups of organisms and it was shown that the propionic acid was formed from the metabolism of phenol. From the nature of the operating strategy, it was deduced that the digester continued to convert all the glucose that was supplied to methane showing that propionic acid accumulation did not inhibit conversion of glucose to methane. Therefore, propionic acid accumulation map be an effect and not a cause of inhibition of the anaerobic digestion process.