Single crystals of the potassium salt (K 5 , H 3 O)[SiV 3 W 9 O 40 H]·xH 2 O of the vanadium tri-substituted α-Keggin dodecatungstosilicate were prepared and analyzed by vibrational, EPR and 51 V NMR spectroscopy. Varying the synthesis conditions allows crystallization of partially reduced anions. The crystal structure was determined for both oxidized (V 5+ ) and partially reduced (V 4+/5+ ) potassium salts. Single crystal X-ray diffraction data and solid state 51 V-NMR spectra confirm the occurrence of a single vanadium site in a cubic structure due to rotational disorder of the Keggin ion. Partially reduced compounds crystallize within the same structure as fully oxidized ones. EPR experiments confirm strong interaction of V 4+ with two V 5+ ions, in accordance with insertion of a V 3 subunit into the lacunary Keggin ion as designed in the synthesis method. The 3D-edifice is composed of K + /H 2 O counter-sublattice with evidence of tunable water occupancy.