Antimony-modified vanadia-on-titania catalysts were prepared for the selective oxidation of o-xylene to phthalic anhydride by ball milling of powder mixtures followed by calcination. A binary Sb2O3-V2O5 system was also prepared for comparison purposes. The resulting materials were physically characterized by surface area measurements, X-ray diffraction analysis (XRD), laser Raman spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, electron spin resonance (ESR), magnetic susceptibility determination, and V-15 solid-state NMR. The catalytic performance of the TiO2-supported materials was tested for o-xylene oxidation. After calcination of the Sb2O3-V2O5 binary mixture at 673 K, Sb3+ is almost quantitatively oxidized to Sb5+, while both V3+ and V4+ are detected. V3+ and some V4+ are most likely located in a nonstoichiometric VSbO4-like structure, while the majority of V4+ preferentially concentrates within shear domains in oxygen-deficient V2O5-x particles. In the titania-supported catalyst system, both Sb2O3 and V2O5 spread on the anatase surface. Sb3+ is oxidized to Sb5+, and V3+, V4+, and V5+ are detected. VSbO4-like structures are not observed. The presence of antimony leads to the formation of presumably V3+-O-V5+ redox couples. The paramagnetic centers-in contrast to the binary mixture-are largely isolated. Antimony preferentially migrates to the surface and appears to exhibit a dual function catalytically. It is inferred from the experimental data that the addition of antimony leads to site isolation and to a reduction of surface acidity. We suggest that V-O-V-O-V domains or clusters are interrupted by incorporation of Sb to form V-O-Sb-O-V species. As a consequence of this site isolation and a reduction of surface acidity, overoxidation of o-xylene is reduced. These two effects are therefore most probably responsible for the improved selectivity of the ternary catalyst system over the binary one toward phthalic anhydride.