Titania-supported vanadia (V/Ti/O) systems were modified by addition of cesium oxide for application as catalysts in the selective oxidation of o-xylene to phthalic anhydride (PA). Catalytic tests demonstrated that cesium is a strong promoter of the activity and selectivity to PA, but this effect is evident only under well-defined reaction conditions. Samples with a Cs content lower than 0.35 wt.% Cs2O exhibited a considerable increase in conversion as compared with the undoped V/Ti/O system. Catalytic tests made with varying o-xylene and oxygen concentrations in the feed demonstrated that in Cs-doped V/Ti/O catalysts the rate-determining step is the re-oxidation of vanadium by molecular oxygen. Thermal-programmed reduction (TPR) and thermal-programmed re-oxidation (TPO) tests evidenced that the addition of Cs decreases the vanadium reducibility and increases the re-oxidizability of the reduced vanadium sites. The positive effect of Cs on selectivity to PA was evident only for o-xylene concentrations in feed lower than 1.5 mol%.