The thermal decomposition of silica-supported phosphomolybdic acid (H3PMo12O40) has been followed by different techniques (TGA-DTA, FT-IR, and Raman spectroscopies, XRD) and methanol oxidation as a probe reaction. When Mg cations are present on the silica surface. the magnesium salt HMgPMo12O40 with Keggin structure is formed in situ on the silica surface during catalyst preparation by impregnation. The presence of Mg induces a two-step thermal decomposition of the Keggin ion by stabilizing an intermediate surface species identified to a planar oxide cluster with the same nuclearity and composition as those of the Keggin precursor. High temperature (greater than or equal to 500 degrees C) calcination leads to the formation of crystallized alpha-MoO3. In absence of Mg, the decomposition occurs in only one step and leads to the formation of alpha- and beta-MoO3 in a ratio depending on the calcination temperature.