The present work deals with the study of the role of promoters in TiO2-supported vanadium oxide, catalyst for the oxidation of o-xylene to phthalic anhydride. Two different series of catalysts were prepared, the first one consisting of undoped samples having different vanadium oxide content, and the second one of samples having 7 wt.% V2O5 and variable amounts of Sb and Cs as promoters. All the samples were characterized by means of Raman spectroscopy, X-ray diffraction and thermal-programmed reduction and oxidation, in order to define a method for the quantification of the different V species (i.e., isolated vanadium, dispersed polyvanadate, and bulk vanadium oxide) that develop on TiO2 support in the presence of promoters. It was found that polyvanadate and bulk vanadium oxide spontaneously release molecular oxygen at 600-650 degrees C, whereas the isolated V is not susceptible of self-reduction. The latter species is predominant in samples having low vanadium oxide loading (<= 2 wt.% V2O5, with TiO2 surface area 22.5 m(2)/g), and possesses the highest intrinsic activity in o-xylene conversion. The presence of Sb, a promoter of activity, increases the dispersion of the most active species and also hinders its segregation in the reaction environment. These promoting effects are more pronounced when both Cs and Sb are present. (c) 2006 Elsevier B.V. All rights reserved.