Thermal stabilities of Wells-Dawson polyoxometalates of overall skeleton composition P2W18O626- were examined and their catalytic properties in isobutane oxidehydrogenation were tested as a function of temperature, isobutane and oxygen content and presence of steam. It is shown that the lacunary K10P2W17O61 and the iron-substituted K7P2W17FeO61 are stable under oxidizing conditions up to 500degreesC. At higher temperature structural modifications occur with breaking of the polyanion framework and formation of K/W/O and K/P/O-containing phases. These rearrangements are thermally induced (they occur under both air and inert atmosphere) and do not involve the formation of the Keggin-type phases, as previously observed with K6P2W18O62. The Wells-Dawson compounds were active and selective in isobutane oxidehydrogenation to isobutene. However, a high activity was evident only under conditions which favoured the polyoxometalate reduction, and the possible generation of radical species which propagated into the gas phase, thus yielding a reactivity typical of heterogeneously-initiated, homogeneous reactions. These conditions were found at temperature higher than 450 degreesC, high isobutane concentration and high oxygen concentration in feed. A specific role of water in the reaction has been found; specifically, in the presence of steam the contribution of homogeneous reactions was enhanced. (C) 2003 Elsevier Science B.V. All rights reserved.