Catalytic performance in o-xylene oxidn. of two samples of vanadium oxide on rutile or anatase TiO2 supports, prepd. by mech. mixing, is analyzed with ref. to their characterization by x-ray diffraction, FTIR and chem. analyses. Results show that the active phase, characterized by a V+5:V+4 ratio of 2:1 and an IR band at 995 cm-1, does not interact directly with the titania surface, but rather with a V+4-modified surface. It is suggested that these V+4 surface sites act as the clinging sites for stabilization of an upper active phase. The same active phase is found on both anatase- and rutile-based samples. Competing migration towards the bulk of these V+4 sites on the rutile sample causes a slight decrease in catalyst performance, in particular, enhanced formation of the intermediate product, phthalide, is obsd. [on SciFinder(R)]