Recent results of Concepcion's group (Chem. Com51 (2015) 4105) on water oxidation catalysis (WOC) by a ruthenium complex suggest that, at pH = 8, O-2 release takes place after formation of a rhomboid bis(mu-oxo)-Ru-2(V) species and not after generation of the typical mu-eta(1):eta(1)-peroxo-Ru-2(VI) intermediate, coming from the coupling of two Ru-V=O moieties (I2M mechanism), which is widely accepted to be formed at pH = 1. To analyze the differences between the reaction mechanisms of this WOC at different pHs, we performed DFT calculations of the full mechanism at pH = 1 and 8 of the WOC process catalyzed by the 2,2'-bipyridine-6,6'-dicarboxylate Ru complex. At pH = 8, we found that barriers leading to the hypothetic formation of rhombic (Ru2O2)-O-V species are higher than those involved in the canonical I2M mechanism. The rate determining step at the latter pH is found to be the dimer formation while the bond cleavage for the O-2 liberation process is barrierless. The computational results confirm that the most common I2M mechanism is preferred at both pHs, as the new proposal comprising formation of bis(mu-oxo)-Ru-2(V) species involves higher energy barriers.