This study involved the catalytic behaviour and the deactivation of aluminum-vanadium-phosphorus (Al-V-P-O) systems obtained by the sol-gel method with several P/Al/V molar ratios and calcined at 450 degrees C in the selective oxidation of o-xylene. Activity values as well as the reducibility of Al-V-P-O systems were influenced by the Al/V ratio, increasing with Al content, while selectivity to phthalic anhydride and phthalide values only changed slightly. All catalysts always showed a high degree of resistance to deactivation in function of time on-stream. Furthermore. water in the feed or coke generation treatment negatively affected catalytic activity, although the initial conversion values were recovered. This fact along with the unchanged textural properties and XRD patterns of the catalysts after such treatments suggest that reversible deactivation could be attributed to partial poisoning of active sites. (c) 2006 Elsevier B.V. All rights reserved.