The chemical and structural changes undergone by the Keggin-type compound Cs(NH4)(2)PMo12O40 with increasing time on stream under the conditions for methacrolein to methacrylic acid conversion have been studied by thermal analysis and in situ X-ray powder diffraction, which allowed the structure of the working catalysts and the changes involved in the catalyst ageing processes to be determined, The results show that, under the operating conditions of the catalytic reaction, restructuring of the (PMo12O40)(3-) Keggin anion takes place, leading to formation of the reduced heteropolyanions (PMo13O40)(-) and (PMo14O42)(-). The six-electron reduced polyanion (PMo14O42)(-) represents the end member of a series of catalytically active and thermally stable species which, by further reduction, decompose into catalytically less active MoO3. Performance of catalysts derived from the above cesium-containing heteropolyacids is also compared to that of the anhydrous acid H3PMo12O40 and of the anhydride (MoOx)(0.5)(PMo14O42) (X similar or equal to 2), for which a detailed structural analysis was carried out.