Cesium and cesium-transition metal-substituted Keggin-type phosphomolybdic acids have been prepared and characterized using several techniques. The results obtained show that all heteropoly-compounds with formulae H3-xCsxPMo12O40 where 0 < x < 3 were composed of two phases corresponding to the hydrated acid and the pure cesium salt with the acid phase coating the salt particles. For compounds with x < 2, the acid was present in large amounts and could be detected by X-ray diffraction. For compounds with x > 2 the precipitation rates vary, leading to very small cesium salt particles. In this second case the acid phase also coated the particles, but it was no longer detectable by X-ray diffraction, although it could be observed by XPS or Raman spectroscopy. All the data obtained show that the transition metal cations are replacing the protons in the supported acid phase and not the cesium atoms in the salt. This replacement does not change the structure of the bulk acid phase even at high substitution levels, but it decreases the hydration rate of the supported acid.