Formaldehyde (FA) degradation in anaerobic batch digesters was studied in the presence and the absence of a cosubstrate composed by volatile fatty acids (VFA). Nonadapted sludges were used with specific maximum methanogenic activities on VFA around 0.6 g chemical oxygen demand (COD)g(-1) volatile suspended solids (VSS)day(-1) and with a significant sulfate-reducing activity which led to the consumption of around 12% COD when sulfate was present. FA degradation was favored by the presence of the cosubstrate especially when high acetate concentrations were used (30 mM vs. 10 mM). Both in the absence and the presence of the cosubstrate, methanol was shown to be key intermediate. FA exerts a clear inhibitory effect on VFA degradation. Values of 4.17 mM (125 mg FA l(-1)) exert a 50% decrease in the specific sludge activity. When FA concentrations ranged from 5-6.67 mM (150-200 mg FA l(-1) ), methanol was accumulated in the medium. Abiotic experiments confirmed that a real FA biological degradation occurred. A pathway for the anaerobic degradation of FA involving methanol as an intermediate product is proposed.