The thermal behavior of 12-molybdophosphoric acid (PMo12H) was investigated using coupled techniques (IR and Raman spectroscopies, polarography, X-ray diffraction, and catalytic reactivity in methanol oxidation) and compared with results obtained with 12-molybdosilicic acid. The decomposition of PMo12H occurs in a wide temperature range, leading to mixtures of beta- and alpha-MoO3, A great analogy between the P and Si compounds is evidenced outside the decomposition range, The differences inside this range (wide for P, narrow for Si compound) are discussed in terms of kinetics of decomposition, textural evolution, and capability to rebuild the Keggin unit,