Abstract Transformations of para-substituted benzonitriles in sediment and in sediment extract were studied. In raw anaerobic sediment, most of the benzonitriles were transformed to benzoic acids without benzamide intermediates. Very slow disappearance of benzonitrile in autoclave-sterilized sediment was explained by the chemical hydrolysis, while relatively rapid decrease in γ-irradiation-sterilized sediment indicated another abiotic mechanism. Hence, extracted protein fraction from sediment active against benzonitrile was surveyed. The obtained active fraction transformed para-substituted benzonitriles to corresponding benzoic acids without any intermediates, which resembled reported nitrilase enzyme activity. Quantitative structure-reactivity relationship (QSRR) analyses of para-substituted benzonitrile transformation-rate constants in raw sediment and in sediment extract fraction indicated that both of them had some correlation with hydrophobicity. This contrasted with the chemical hydrolysis of these compounds, which was controlled by electronic substituent constant (Hammett σp). This was an encouraging sign that the obtained sediment extract fraction is possibly responsible at least for part of the reaction in raw sediment. Various reaction mechanisms are present for degradation of xenobiotics in anaerobic sediment. This study presented the possible abiotic mechanism attributable to protein fraction in sediment. Further studies are necessary to understand the total reaction mechanisms in aquatic sediment.