A [iPrNH3]+/Na+-mixed salt of unprotonated two-electron-reduced decatungstate, [iPrNH3]8[W10O32]·(NaCl2)2H2O (1), is isolated from the photolyte obtained by the UV photolysis of the Na2WO4·2H2O/[iPrNH3]Cl aqueous solution at pH 6.5 and characterized in the solid state (X-ray diffraction and elemental analysis) and solution (UV-vis absorption and electrochemistry). [W10O32]5-, as a paramagnetic contaminant in the crystallite of diamagnetic 1, shows only a broadened singlet-like electron spin resonance (ESR) line around g = 1.82 with an unresolved 183W-hyperfine interaction (AW) at low temperatures. To determine the effective electronic delocalization in the reduced decatungstates consisting of two polar WO6 (WpO6) and eight equatorial WO6 (WeqO6) octahedra with an approximate D4h symmetry, we calculated isotropic ESR spectral patterns of the protonated reduced decatungstate, [HW10O32]4-, for a variety of |AW| on the basis of interactions of the 5d1-electronic spin with eight magnetically equivalent nuclear spins of Weq and the proton 1H (IH = 1/2) and took |AW| = 2|AH| (AH = 1H-superhyperfine interaction) as the best choice for direct comparison with the experimental single-crystal ESR spectral pattern of [HW10O32]4- at 77 K. Electronically fully delocalized over eight WeqO6 octahedron mixed-valence sites of class-III-type clusters, [W10O32]5- ≡ [Wp6+2(Weq5.875+)8O32]5- and [W10O32]6- ≡ Wp6+2(Weq5.75+)8O32]6-, are demonstrated with the help of ESR spectroscopy of [HW10O32]4- ≡ [Wp6+2(H0.125Weq5.875+)8O32]4- which revealed that both the 5d1-electron and proton are situated at each equatorial WO6 octahedron site with a 1/8 occupancy at low temperatures (≤77 K). The nucleation process to [W10O32]6- through the UV photolysis of [W7O24]6- at pH 6.5 is also discussed based on the electrospray ionization mass spectrometry of the photolyte.