Abstract: A process for treating waste water containing both phenolic compounds being toxic for methanogenic organisms and degradable non-phenolic compounds prior to its anaerobic purification as well as a process for detoxifying waste water containing phenolic compounds being toxic for aquatic organisms by subjecting said waste water to an oxidative treatment in such a way that no or virtually no precipitation is formed of the polyphenol compounds produced and virtually no influence is executed on the biochemical oxygen demand and the chemical oxygen demand of the waste water.