The thermal and structural stability of the Wells-Dawson-type heteropoly compound K6P2W18O62. 10H(2)O was examined by FT-IR spectroscopy, X-ray powder diffraction, thermogravimetric analysis and HRTEM. It was found that calcination at temperatures higher than 850 K led to the formation of a Keggin-type compound "K3PW12O40", containing small amounts of an additional phase originated from the high-temperature interaction between potassium phosphate (K3PO4, formed during the decomposition of the K6P2W18O62. 10H(2)O) and the Keggin-type compound itself. The Keggin-type product showed a higher activity in the selective oxidative dehydrogenation of isobutane to isobutene compared to both the Wells-Dawson precursor and to pure, authentic K3PW12O40. This higher activity can be tentatively attributed to the presence of an amorphous layer of unknown stoichiometry at the surface of the thermally rearranged Wells-Dawson compound.