The Keggin decamolybdodivanadophosphate ions have been studied in aqueous 0.6 M Na(Cl) medium by potentiometry and P-31 (202.5 MHz) and V-51 (131.55 MHz) NMR spectrometry. An assignment of all detected resonances of decamolybdodivanadophosphate species to individual Keggin isomers is proposed. The rive positional isomers of alpha-[Mo10V2PO40]5- were identified and characterized. Two minor [Mo10V2PO40]5- species, probably beta-Keggin isomers, were also observed. The pK(a) values of the conjugate monoprotonated species, [HMo10V2PO40]4-, have been precisely determined from the P-31 and V-51 chemical shift vs pH behavior, using alpha-[Mo11VPO40]4- as internal standard. These pK(a) values are in the range 0-0.5 at 25-degrees-C and in the range 0.8-1.1 at 90-degrees-C. From quantitative NMR integral data the relative abundances of the alpha isomers at equilibrium were evaluated. The distribution among them is close to statistical, with little measurable change between 25 and 90-degrees-C.