Biotreatment experiments with solutions of autoxidized phenolic compounds as well as coal-conversion wastewater stored for 30 years and rich in humic matter were performed under nitrate-reducing, sulphate-reducing and methanogenic conditions. The removal of total organic carbon in fractions of different molecular mass and of monomeric phenolic compounds in the wastewater was determined. A comparison of biotransformation potentials and rates indicated a relationship between these aspects and the availability of electron accepters in the system. The capacities of the microbial consortia increased significantly with the energy microorganisms could gain from their respective respiration process and can be expressed by the order : aerobic process - nitrate reduction - sulphate reduction - methanogenesis.