In this paper, we report the preparation of caesium-exchanged phosphomolybdic acid, Cs3PMo12O40, via the solid-state reaction between caesium chloride and ammonium phosphomolybdate. The calcination of this mixture was performed at 400 degrees C-600 degrees C. The thermal genesis of Cs3PMo12O40 from its precursors was explored using thermo-gravimetric (TGA), differential thermal (DTA), Fourier transform infrared (FIR) spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray analyses. The results obtained indicated that pure ammonium phosphomolybdate hydrate first lost its crystal water and later decomposed at temperatures higher than 450 degrees C. In the CsCl and (NH4)(3)PMo12O40 mixture, it was demonstrated that Cs+ moves into exchange positions at such temperature without destroying the Keggin structure of the heteropoly compound. (C) 2013 Elsevier B.V. All rights reserved.