The active sites of samarium orthovanadate (SmVO4) were studied by means of ESR, NO-TPD and temperature programmed O-18(2)-isotope exchange (TPIE) methods. The results of ESR and NO-TPD confirm the presence of V4+ in the catalyst. The TPIE revealed that the O-18(2)-isotope exchange was carried out through a single exchange procedure. The V4+ species associated with oxygen vacancies are the sites for O-2 activation