Three Keggin-type heteropolyanions,namely H_3PMo_(12)O_(40)·13H_2O,(NH_4)_3PMo_(12)O_(40)·4H_2O and H_3PW_(12)O_(40)·13H_2O were prepared and tested in the ring-opening polymerization reaction of tetrahydrofuran.The effects of the counter-cation (H~+,NH_4~+) and the peripheral atoms(Mo,W) on the polymerization were investigated.It has been found that when the protons of H_3PMo_(12)O_(40)·13H_2O were replaced by the ammonium cations the polymerization rate decreased dramatically. Whereas,when the peripheral atoms(Mo) were replaced by their homologous(W),the polymerization rate increased twofold.As for the viscosity average molecular weight(M_v) of polymer products,it was found that the high molecular weight(7930) was obtained by using H_3PW_(12)O_(40)·13H_2O.The molecular weight(M_v) obtained by H_3PMo_(12)O_(40)·13H_2O and (NH_4)_3PMo_(12)O_(40)·4H_2O was 6470 and 6810,respectively. Three Keggin-type heteropolyanions, namely H_3PMo_ (12) O_ (40) · 13H_2O, (NH_4) _3PMo_ (12) O_ (40) · 4H_2O and H_3PW_ (12) O_ (40) · 13H_2O were prepared and tested in the ring- opening polymerization reaction of tetrahydrofuran the effects of the counter-cation (H ~ +, NH_4 ~ +) and the peripheral atoms (Mo, W) on the polymerization were investigated. It has been found that when the protons of H_3PMo_ (12) Whereas, when the peripheral atoms (Mo) were replaced by their homologous (W), the polymerization rate increased twofold. As for the viscosity average molecular weight (O) (40) · 13H_2O were replaced by the ammonium cations the polymerization rate decreased dramatically. M_v) of polymer products, which was found that the high molecular weight (7930) was obtained by using H_3PW_ (12) O_ (40) · 13H_2O.The molecular weight (M_v) obtained by H_3PMo_ (12) O_ (40) · 13H_2O and (NH_4) _3PMo_ (12) O_ (40) · 4H_2O was 6470 and 6810, respectively.