The reducibility and isotope exchange ability of different oxygen bonds (O_b, O_p, O_t) in the Keggin structure of heteropolymolybdates (H_(3+n)PMo_(12-n)V_nO_(40)) containing zero, two or four vanadium atoms as well as the changes of structure of heteropolyacids during reduction were investigated by IR, ESR and ~(18)O isotope exchange. The results obtained showed that the order in the reducibility of different oxygen bonds is O_b>>O_p≈O_t, the replacement of Mo by V affects obviously the reducibility of various oxygen bonds: O_b increases while O_p and O_t decrease. ~(18)O exchange rate of O_b, O_p, O_t is not different, comparatively speaking, the isotope exchange of oxygen of the heteropolyacids containing vanadium atoms is more difficult than PMo_(12). The ESR spectra indicate that two kinds of Mo~(5+) ions with different coordinations are present in PMo_(12) (n=0) during reducing process, their g values are g_⊥~A=1.971, g_(?)~A=1.912 and g_⊥~B=1.965, g_(?)~B=1.861 belonging respectively to the d The reducibility and isotope exchange ability of different oxygen bonds (O_b, O_p, O_t) in the Keggin structure of heteropolymolybdates (H_ (3 + n) PMo_ (12-n) V_nO_ (40)) containing zero, two or four vanadium atoms as well as the changes of structure of heteropolyacids during reduction were investigated by IR, ESR and ~ (18) O isotope exchange. The results obtained showed that the order in the reducibility of different oxygen bonds is O_b >> O_p≈O_t, the replacement of O (18) O exchange rate of O_b, O_p, O_t is not different, comparatively speaking, the isotope exchange of oxygen of the heteropolyacids containing vanadium The ESR spectra indicate that the two kinds of Mo ~ (5+) ions with different coordinations are present in PMo_ (12) (n = 0) during reducing process, their g values ​​are g_⊥ ~ A = 1.971, g _ (?) ~ A = 1.912 and g_⊥ ~ B = 1.965, g _ (~) ~ B = 1.861 respectively to the d