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The pure phase of praseodymium orthovanadate (PrVO-4) has been prepared by the citrate method. The active site of PrVO-4 was studied by ESR, NO_TPD, O-2_TPD and 18 O-2_isotope exchange methods. The results of ESR and NO_TPD confirmed the presence of V 4+ in the catalyst. 18 O-2_isotope exchange was through a single exchange procedure. From the result of O-2+TPD and the kinetic study of 18 O-2_isotope exchange, one can reach a conclusion that the V 4+ species associated with oxygen vacancies are the site for O-2 activation. The adsorbed O-2 or O+- are the active oxygen species in propane oxidative dehydrogenation.