Water exchange on a molecular, purely inorganic cobalt-based water oxidation catalyst, [Co-4(II)(H2O)(2)(alpha-P1W9O34)(2)](10-) (1), in the catalytically relevant pH region (pH 6-10) is studied using O-17-NMR spectroscopy and ultrahigh-resolution electrospray ionization mass spectrometry. The results are compared with those of the inactive [Co-II(H2O)(1)Si1W11O39](6-) (2.), which is stable in the same pH region. The results obtained provide mechanistic details of the elementary reaction step related to the water oxidation on homogeneous metal oxide catalysts under catalytically relevant conditions. It is shown that the structural integrity of 1 and 2 is maintained, no deprotonation of the aqua ligands on the Co(II) centers occurs, and the water exchange does not undergo any mechanistic changeover at the catalytic pH conditions. We have demonstrated that the water exchange process is influenced by the cluster environment surrounding the water binding sites and is fast enough to not be rate-limiting for the water oxidation catalysis.