The interaction of oxygen with sublimed layers of meso-monopyridyltriphenylporphyrinatocobalt(II) (CoMPyTPP) was studied by electronic absorption and IR spectroscopy, The reversible binding Of O-2 to the axial 5-position was shown for the freshly sublimed layers: the 6-position can either be free, or occupied by the pyridyl group of the adjacent molecule in the layer. The concentration of the complexes of the first type decreases to zero with time, whereas that of the second type complexes increases. The binding of oxygen on the sublimed layers of CoMPyTPP differs substantially from that on meso-tetraphenylporphyrinatocobalt(II) and meso-tetra-3(4)-pyridylporphyrinatocobalt(II), which loose their capability of O-2 binding with time.