This paper discusses the applicability of a low-cost method for evaluating the ability of anaerobic micro-organisms to decompose phenolic compounds found in olive mill wastewaters. Gallic acid (GA) was used as test substance at concentrations of 100, 500 and 1000 mg L-1. When 1000 mg L-1 GA was inoculated with anaerobic sludge, the average (of six replicates) cumulative gas volume collected over a period of 23 days was 87.6 +/- 2.4 mL, while without sludge the volume was 9.3 +/- 1.0 mL. The corresponding values for 500 mg L-1 GA were 48.0 +/- 8.0 and 6.5 +/- 1.0 mL respectively, while for 100 mg L-1 GA they were 30.8 +/- 4.7 and 9.6 +/- 2.3 mL respectively. These results suggest that gallic acid is readily degradable under anaerobic conditions even at concentrations as high as 1000 mg L-1 if adequate fermentation time and appropriate microbial culture are provided. The gas volume produced was found to depend linearly (r(2)= 0.95) on the concentration of gallic acid under the conditions studied. (c) 2006 Society of Chemical Industry.