The resonance Raman spectra of dioxygen adducts of Co(sal-Hpen) and its methoxy derivative, Co(sal-mpen) were measured in methylene chloride solution at ∼−90 °C. Co(sal-mpen)O 2 exhibits the ν(O 2) at 1148 cm −1, typical of six-coordinate Co(salen)(base)O 2 type complexes, whereas Co(sal-Hpen)O 2shows the ν(O 2) at 1095 cm −1, which is close to that of ‘base-free’ Co(salen)-O 2 (1097 cm −1). These results indicate that pendant methoxyphenyl group of Co(sal-mpen)O 2 is coordinated to the axial position whereas the pendant hydroxyphenyl group of Co(sal-Hpen)O 2 is free from such interaction. When anisole is added to a solution of Co(salen)-O 2, a new ν(O 2) band due to Co(salen)(anisole)O 2 appears at 1145 cm −1. No such band is observed, however, when phenol is added to the same solution. The phenolate ion (OR −) tends to form Co(III) complexes Co(III)(salen)(OR −) which yield peroxo dimers, [{Co(III)(salen)(OR −)} 2O 2] 2− (ν(O 2), ∼805 cm −1) upon oxygenation.