The iron(III) complex of protoporphyrin IX is the basic component of the active sites of cytochromes P-450 and related mono oxygenases. The axial ligand in cytochromes P-450 is a cysteine thiolate, whereas in peroxidases it is the imidazole of a histidine residue of the protein. During the reaction sequence of peroxidase, the iron(Ill) porphyrin moiety of peroxidase reacts with H 2 02 and other mono oxygen donors to give compound I, which, on further one-electron transformation, leads to the formation of an iron(IV) oxo species, compound II of peroxidase. The formation of iron-oxo intermediates, similar to compound I, have been proposed for cytochromes P-450 catalyzed oxidations, except that the axial thiolate ligand favours the formation of the high-valent iron(V) species. Iron-porphyrins and different mono oxygen donors mimic the different reactions of cytochromes P-450 and other mono oxygenases. The formation of the M-O bonds from different metalloporphyrins and mono oxygen donors has been proposed in the hydroxylation of 1,3-dimethylpyrimidines, the epoxidation of aldrin and related olefins as well as in the oxidation of various sulphur compounds. The formation of M-N and M-C bonds has been proposed during the oxidation of different alkylhydrazines and related compounds. The formation of different intermediates and final products are briefly highlighted in the presentation.