The acidity (pull) and the axial ligand (push) effects on the O-O bond cleavage in the [(Salen)Mn-III(RCO3)L] acylperoxo complexes, with model L = none, NH3, and HCO2- (1), have been studied with B3LYP density functional calculations. The acidic conditions have been mimicked by explicit protonation of 1 to afford a variety of [(Salen)Mn-III(RCO3H)L] (2) and [(SalenH)Mn-III(RCO3)L] (3) complexes in ground quintet states. The protonation assists the O-O bond heterolysis, thus primarily forming highly reactive Mn-V(O) species, and consequently suppresses formation of the less reactive Mn-IV(O) species through homolytic channel described earlier in 1 [Khavrutskii, I. V.; Rahim, R. R.; Musaev, D. G.; Morokuma, K. J. Phys. Chem, B 2004, 108, 3845-3854]. In addition to the qualitative change of the O-O bond cleavage mode, the protonation affects the rate of the O-O bond cleavage. Therefore, varying the acidity of the reaction media helps control the O-O bond cleavage mode and rate.