The mechanisms of the oxide syntheses to PMo12 and PVMo11 were enlightened by in situ UV/VIS and P-31-NMR. Different solution stabilities of the products were discovered. After several crystalline phase transformations of the solid at higher temperatures (> 573 K) spectroscopic assignments for defect HPA formation were determined. Water supports reversible restructuring into the intact Keggin structure. As a consequence we propose a model concept for a solid repair cycle of the catalyst.