The degree of reduction/oxidation of H4PVMo11O40 (HPA) and Cs2H2PVMo11O40 (CsPA) was studied during reduction and reoxidation by methanol, ethanol and mixtures with oxygen, respectively. The peak intensity of the intervalence charge transfer (IVCT) band, the apparent band gap energy (E-g*) and catalytic data were obtained by in situ UV-VIS diffuse reflectance spectrosccopy (UV-VIS-DRS) and on-line gas chromatography (GC), respectively. The peak intensity of the IVCT band and E-g* increase during the reduction of heteropoly compounds by the alcohols. The spectroscopic and catalytic data (conversion, selectivity) correlate in the transient state during the reoxidation process. It is shown that isolated Keggin anions act as precursors for the active states of the catalysts, which molecular structure cannot be deduced from UV-VIS spectroscopy alone. UV-VIS spectroscopy, however, can serve as a tool to determine the degree of reduction in future combined in situ UV-VIS/Raman/XRD studied.