Under anaerobic conditions, bacteria convert a multitude of monoaromatic compounds into the central intermediate benzoyl-CoA, which is reduced to a cyclic dienoyl-CoA product by the key enzymes of anaerobic aromatic degradation, benzoyl-CoA reductases. They catalyse the highly endergonic reduction of the aromatic ring to a cyclic diene at a redox potential of E°‘=-622 mV, far below the limits of any natural electron donor. Class I benzoyl-CoA reductases from facultatively anaerobic bacteria solve this problem by coupling aromatic ring reduction by a reduced ferredoxin to a stoichiometric ATP-hydrolysis. Catalysis at an active site [4Fe-4S] cluster proceeds in a Birch-like reduction mechanism via radical intermediates. We are heterologously producing the extremely oxygen-sensitive ATP-dependent benzoyl-CoA reductases and are currently elucidating the structure/function relationship by using protein crystallography, and various kinetic and spectroscopic methods.