In this work, the effect of vanadium loading as an active phase, cesium and antimony as promoters for the performance of Ti-phosphate-supported vanadium(v) oxide catalysts by experiment design method has been investigated. To find out the effects of parameters on the selectivity to phtalic anhydride, Taguchi approach with L16 array and ANOVA analysis was utilized. The catalyst samples were characterized by BET 'XRD"TGA"SEM' NH3-TPD. In addition, the catalytic measurements were carried out under continuous flow conditions in fixed-bed reactor. The results show that cesium improves not only activity but also selectivity to phtalic anhydride, while antimony just improves the activity. Meanwhile, the antimony itself has a negative effect on the selectivity to phthalic anhydride. By taking into account main factor effect plot, the sample with composition of 15 wt% vanadium, 0.35 wt% cesium and 1 wt% antimony was chosen as the optimum sample. Finally, this catalyst demonstrated 70.9% selectivity to phthalic anhydride and 100% conversion of o-xylene at 410 degrees C. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.