The selective oxidation of propane by oxygen has been investigated on a series of gallium modified Cs2.5H1.5PV1Mo11O40 samples, with Ga content varying from 0 to 0.32 per Keggin unit (KU). The physicochemical properties of the samples have been studied by using a variety of techniques, namely ICP, FTIR, XRD, P-31-NMR, BET and TG-DTG-DSC. The effect of several parameters, such as pre-treatment temperature, reaction temperature, residence time (W/F) on the conversion and product distribution has been examined. The presence of gallium has been observed to increase surface area by a factor of more than two and to improve the selectivity to oxygenates (acrylic and acetic acids, acrolein). It has been found that there is an optimum value of Ga content of 0.16 per KU in the Keggin structure and of pre heating temperature of 300 degrees C for maximizing propane conversion and acrylic acid selectivity. The latter aspect is related to the presence of both Bronsted acid and redox sites. (c) 2006 Elsevier B.V. All rights reserved.