A manganese(II) complex with pentadentate ligand derived from 2,6-diacetylpyridine and triethylenetetramine exhibits high activity for activation of dioxygen in the presence of several aliphatic aldehydes, but the activities of other manganese(II) compounds with tripod-like ligands such as tris(2-benzimidazolylmethyl)amine or bis(2-benzimidazolylmethyl) ether are almost negligible. On the basis of these facts including electrochemical data under dioxygen atmosphere, the importance of an intermediate complex formation among aliphatic aldehyde, Mn(II) complex, and dioxygen is pointed out; in the intermediate, the dioxygen molecule is assumed to coordinate to a manganese(II) ion by hydrogen bonding with the hydrogen atom of the chelate (-NH group), and the novel reactivity of dioxygen, i.e., the dioxygen molecule accepts an electron from the aldehyde through formation of the intermediate to yield an organic peracid without the change of the oxidation state of the metal ion, is postulated.