Two sets of homologous Mn-and Fe-catalysts, [(MnLCl2)-L-II], [(FeLCl2)-L-II] and [(MnL)-L-II(OAc)(2)], [(FeL)-L-II(OAc)(2)] have been synthesized. A detailed comparative study of their catalytic oxidative performance with H2O2, in tandem with EPR and Low-Temperature UV-vis spectroscopies has been carried out. The [Metal-L (OAc)(2)] and [Metal-LCl2] catalysts did not show any difference in their catalytic behavior i.e. there is no effect of the labile ligands on the studied catalysis. It is found that the Mn-catalysts consistently outcompeted the homologous Fe-catalysts i.e. TOFs (Mn) =162 vs. TOFs (Fe) = 16. We found that the Fe-catalyst faces a significantly higher activation barrier than the Mn-catalyst i.e. E-a((FeL)-L-II(OAC)(2)) = 91KJ/mol >> Ea((MnL)-L-II(OAC)(2))= 55 kJ/mole, while the free-energy difference, Delta G((FeL)-L-II(OAC)(2)) similar to Delta G (Mn-II(OAC)(2)) similar to-145 kJ/mole, did not make difference. Taken altogether the present data clarify that the main thermodynamic barrier, ultimately determining the overall catalytic performance, of these homologous Mn-and Fe-catalysts is the activation energy for the transient intermediates i.e. Mn-II to Mn-IV=0 for the Mn-catalysts and Fe-II to Fe-II-OOH for the Fe-catalysts. A unified/consistent catalytic thermodymanic concept is discussed, that bears relevance to the catalytic behavior of many non-heme Mn-vs. Fe-oxidation catalysts. (C) 2016 Elsevier Inc. All rights reserved.