Model systems were used to investigate the mechanism of enzymatic hydroxylation. It could be shown that hydroxylation of aromatic groups with molecular oxygen takes place not only in the well known Fe2⊕/EDTA/ascorbic acid system, but also in all systems which have a suitable “Redoxpotential”, or ability to reduce oxygen. As well as the Fe2⊕/EDTA complex, Cu⊕, Ti3⊕, Ti3⊕/EDTA, V3⊕ and Sn2⊕ are active. In the case of hydroxylation of acetanilide, it could be shown that for systems which activate molecular oxygen, other o-, m-, p-ratios of hydroxyacetanilide appear than in systems which have OH radicals as hydroxylating particles. Participation of O2H radicals as well as OH radicals in hydroxylation is probable.