Abstract: The ESR spectra of five reduced heteropolyanions belonging to the Keggin structure are reported in this paper. They are SiV₂Mo₁₀O₄₀^6-, SiV₃Mo₉O₄₀^7-, PV₃MO₉O₄₀^6-, PV₃W₉O₄₀^6- and VV₃W₉O₄₀^6-. At room temperature and pH≤ 3, their spectra contained 8 hfs lines, while at pH=7-8, 8+15 hyperfine lines. Their shape change gradually and finally turned to 8-lines spectrum after a long period of time. The frozen spectra at - 150℃ exhibited complicated 8 line spectra which were successfully simulated based on slightly modified Bleaney's theory.The ESR parameters g_‖ g_⊥ A_‖ A_⊥ were determined and the approximate K, α values were calculated based on DeArmond's theory. The physical meaning of the results are discussed and the failure of observing the 22 hfs lines of V_s species are primarily explained according to the number and degeneracy of the isomers present in the V₃ species.

Key words: ESR, heteropolyanions, heteropoly-blue