The influence of K, Ba, Bi, W and Sn oxides or carbonates on catalytic properties of vanadia/titania in the oxidation of o-xylene to phthalic anhydride has been studied in a recycle reactor. In general, conversion and selectivity were strongly influenced by pretreatment conditions (temperature, nature of atmosphere). A restructuring of the catalysts was observed during o-xylene oxidation, resulting in higher V/Ti surface ratios, without changing crystallinity or crystal size. Acid-base properties of the catalysts were characterised by ammonia adsorption and temperature-programmed desorption, combined with FT-IR and mass spectroscopic analysis. At temperatures higher than 370-degrees-C, good correlation was observed between the strength of acid sites (K < Ba almost-equal-to Bi < unpromoted almost-equal-to W almost-equal-to Sn) and phthalic anhydride yield (K < Ba < Bi < unpromoted almost-equal-to W almost-equal-to Sn). At low conversion and temperature only basic promoters had a positive influence. Redox properties were studied by TPR and XPS analysis. The oxidation state of vanadium was + 5 before and after reaction and it was independent of promotion. SnO2 and Bi2O3 improved the reducibility of vanadia by hydrogen, but no general correlation could be observed between redox properties and catalytic features.