Modification of the Co-oxo cores of cobalt-polyoxometalate water oxidation catalysts is detectable by X-ray absorption spectroscopy (XAS) as demonstrated by comparison of Na10[Co4(H2O)2(PW9O34)2] (1) and Na17[((Co(H2O))Co2PW9O34)2(PW6O26)] (2). XAS reveals the integrity of 1 uncompromised by oxidant-driven water oxidation, which proceeds without formation of catalytic cobalt oxide.