Heteropolycompounds of the composition Kx(NH4)3-xPMo12O40 were prepared by precipitation, through addition of HNO3 to an aqueous solution of the salts. The precipitated compounds were calcined at increasing temperatures to evaluate the thermal behavior, and were characterized by means of X-ray diffraction, FTIR spectroscopy, surface area measurements, ionic chromatography, EPR spectroscopy, and reactivity in the oxidative dehydrogenation of isobutyric acid to methacrylic acid. A monophasic system was obtained by calcination at 640 K, with patterns characteristic of the Keggin-type cubic secondary structure. The cationic composition was found to be very close to that expected for the complete salification of the Keggin anion. The structural decomposition began at temperatures close to 693 K for the ammonium and ammonium/potassium compounds, with formation of an intermediate heteropolycompound characterized by crystallographic parameters and cationic composition different from the original one and more stable than the latter. The potassium salt remained instead structurally intact up to 753 K. The data were interpreted with the assumption that the decomposition of the heteropolycompound might occur in a stepwise way, with an initial structural collapse of a part of the compound and displacement of some molybdenum ions (originally located in peripheral position in the Keggin anion) in the cationic position of the not yet decomposed compound. The highest amount of this intermediate molybdenum-salified heteropolycompound apparently formed during decomposition of the (NH4)3PMo12O40 compound. The calcination temperature remarkably affected the catalytic behavior in the oxidative dehydrogenation of isobutyric acid to methacrylic acid. The partial structural collapse occurring by calcination at 693 K led to a decrease in the activity for the potassium/ammonium mixed salts. On the contrary, the ammonium salt increased its activity. It was assumed that this phenomenon could be due to the formation of the intermediate molybdenum-salified heteropolycompound.