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By the reaction of MnII(salen) (salen=N,N′-ethylenebis(salicylideneiminato) dianion) and its analogues with molecular oxygen in organic solvents, three types of complexes including as structural units the MnIII–O2–MnIII,–(–MnIV–O–)–n, and MnIV=O bonds respectively, have been obtained, and characterized by means of their electronic and infrared spectra, by thermogravimetric analysis, and in terms of their magnetic properties. The lower magnetic moments for [MnIII(salen)]2O2 and –[–MnIV(salen)–O–]–n than those expected have been interpreted in terms of antiferromagnetic interactions. The oxo complex was found to give the catena-oxo complex quantitatively in a DMF, DMSO, or Py solution. The effects of the oxygen partial pressure and the nature of the substituent in the organic part on the complex formation were investigated. A lower partial pressure of oxygen has the advantage of yielding the μ-peroxo complex in a higher yield. A discussion of the reaction mechanism is also included.