In the present work we report the reactivity of [LMnII](2+) toward addition of hydrogen peroxide (H2O2) in acetonitrile solution, where L is a pentadentate polypyridine ligand. Formation of peroxo complexes is evidenced by low-temperature UV-visible spectroscopy, ESI-mass spectrometry, and EPR spectroscopy using parallel as well as perpendicular mode detection. The influence of the medium (basicity, water content) on the formation of various species is investigated. In basic nonanhydrous medium the fate of the reaction mixture solution is the formation of the di-mu-oxo mixed-valent Mn(III)Mn(IV) dinuclear complex. In acidic medium the building of the oxo bridges is avoided and the reaction mixture evolves toward oxidation of the ligand L. This reaction route offers new opportunities for the study of oxidation reactivity of Mn (hydro)peroxo complexes.