Two series of Keggin-type polyoxometalates (i.e., CsxH3-xPMo12O40 and CsxH4-xPVMo11O40) with different cesium contents have been studied for the oxidative dehydrogenation of propane at a mild temperature (653 K). The conversion of propane decreases while the selectivity to propylene increases with increasing cesium content, and the yield of propylene reaches a maximum at appropriate cesium content for either series of catalysts. The best yield of propylene (9.3%) has been achieved over the CsxH3-xPMo12O40 catalyst with art x value of 2.56, and the vanadium substitution cannot enhance the maximum yield of propylene. The correlation between catalytic behaviors and physicochemical properties of the catalysts suggests that the acidity of the present catalysts plays crucial roles in the oxidative dehydrogenation of propane. It is clarified that the rate of propane conversion increases proportionally to the concentration of Bronsted acid sites, whereas the selectivity to propylene decreases with increasing the surface acidity. (c) 2008 Elsevier B.V. All rights reserved. ; National Basic Research Program of China [2005CB221408]; National Natural Science Foundation of China [20433030, 20773099, 20625310]; Fujian Province of China [2005HZ01-3]