The aim of present Density Functional Theory (DFT) study is the modeling of an activation of molecular oxygen on the manganese porphyrin and a simulation of a set of subsequent processes of hydrogen adsorption, which can lead to the formation of the reactive high-valent manganese-oxo species. Results of the performed calculations reveal that the molecular oxygen may be bound to the reduced manganese porphyrin forming either superoxo or peroxo group. In each case the O-2 Molecule is activated. The adsorption of the hydrogen atom on the O-2 molecule leads to the weakening of the O-O bond. The latter is cleaved once the second hydrogen atom becomes adsorbed. As a result the water molecule is formed. The remaining oxygen atom is doubly bound to manganese ion resulting in the high-valent oxo complex. (c) 2007 Published by Elsevier B.V.