The performance of Mn-II/III-monosubstituted heteropolytungstates [Mn-III(H2O)GeW11O39](5-) ([GT-Mn-III-OH2](5-), where GT = GeW11O39) and [Mn-II(H2O)GeW11O39](6-) ([GT-Mn-II-OH2](6-)) as water oxidation catalysts at pH 9 was explored using density functional theory calculations. The counterion effect was fully considered, in which five and six Na+ ions were included in the calculations for water oxidation catalyzed by [GT-Mn-III-OH2](5-) and [GT-Mn-II-OH2](6-), respectively. The process of water oxidation catalysis was divided into three elemental stages: (i) oxidative activation, (ii) O-O bond formation, and (iii) O-2 evolution. In the oxidative activation stage, two electrons and two protons are removed from [Na-5-GT-Mn-III-OH2] and three electrons and two protons are removed from [Na-6-GT-Mn-II-OH2]. Therefore, the Mn-IV-O-center dot species [Na-5-GT-Mn-IV-O-center dot] is obtained. Two mechanisms, (i) water nucleophilic attack and (ii) oxo-oxo coupling, were demonstrated to be competitive in O-O bond formation triggered from [Na-5-GT-Mn-IV-O-center dot]. In the last stage, the O-2 molecule could be readily evolved from the peroxo or dinuclear species and the catalyst returns to the ground state after the coordination of a water molecule(s).