The inhibitory effects of six kinds of aromatic compounds (benzoate, phenol catachol, resorcinol, hydroquinone, phloroglucinol) on the acetotrophic methanogenesis were investigated by batch anaerobic toxicity assays. The relative methanogenic activity remaining after addition of aromatic compounds decreased with an increase in the concentration of all aromatic compounds tested. The concentrations (IC50) of benzoic acid, phenol, catechol, resorcinol, hydroquinone and phloroglucinol resulting in 50% inhibition of methanogenic activity were 49.2, 18.6, 22.7, 27.7, 40.0, and 51.6 mmol·l-1, respectively. The inhibitory effect caused by aromatic compounds varied with the kind of substituent, as well as the number and position of the substituent in the benzene ring. The toxicity of phenol with OH substituent was greatly higher than that of benzoic acid with COOH substituent. The toxicity of hydroxyl group compounds was found to decrease with increase of the number of OH substituent. For dihydroxy-benzenes, the relative toxicity of isomer decreased in the order of ortho>meta>para. For all the phenolic compounds tested, a good correlationship between their inhibition effect and hydrophobicity was observed, and the logarithm of the octanol/water partition coefficient (log P), an indicator of hydrophobicity, was found to be correlated with the inhibition of methanogenic activity, according to the following eqution : logIC50=-0.283logP+1.68 (n=5, r2=0.953).