In the last 5 years several cobalt-polyoxometalates (Co-POMs) have been reported as all-inorganic water oxidation catalysts (WOCs) with enhanced stability, because oxidative ligand decomposition is not possible. However, while they are promising homogeneous WOCs, they have been highly debated with regard to their stability under turnover conditions. For this reason, these systems are interesting subjects for speciation and stability studies using X-ray scattering techniques – not broadly used in the WOC field. Here we rigorously characterize the speciation of Co-POMs under catalytically relevant conditions. Our studies reveal that: 1) the clusters remain largely intact during reaction with a chemical oxidant, especially in the absence of phosphate buffer; and 2) common phosphate buffers have the greatest effect on the degradation of the Co-POMs under these conditions. This study of the structure in the aqueous state under various conditions provides invaluable information in the understanding of the solution behaviour of Co-POMs and in the development of the next generation of WOCs.