Protons play an important role in promoting O-O or M-O bond cleavage of metal-peroxo complexes. Treatment of side-on O 2 -bound [PPN][Mn IV ( TMS PS3)(O 2 )] (1, PPN = bis(triphenylphosphine)iminium and TMS PS3H 3 = 2,2′,2′′-trimercapto-3,3′,3′′-tris(trimethylsilyl)triphenylphosphine) with perchloric acid (HClO 4 ) in the presence of PR 3 (R = phenyl or p-tolyl) results in the formation of neutral five-coordinate Mn III (OPR 3 )( TMS PS3) complexes (R = phenyl, 2a; p-tolyl, 2b), which are confirmed by X-ray crystallography. Isotope labelling experiments demonstrate that the oxygen atom in the phosphine oxide product derives from the peroxo ligand of 1. Reactions of 1 with weak proton donors, such as phenylthiol, phenol, substituted phenol and methanol, are also investigated to explore the reactivity of the Mn IV -peroxo complex, leading to the isolation of a series of five-coordinate [Mn III (L)( TMS PS3)] − complexes (L = phenylthiolate, phenolate or methoxide). Mechanistic aspects of the reactions of the Mn IV -peroxo complex with proton donors are discussed as well.