Two purple crystals, each containing the tetraperoxovanadate(V) anion, have been characterized by low-temperature X-ray diffraction methods. Na-3[V(O-O)(4)]. H2O2 . 10.5H(2)O crystallizes in the triclinic system, space group P (1) over bar, with a = 9.139(3) Angstrom, b = 11.774(3) Angstrom, c = 9.143(3) Angstrom, alpha = 105.47(2)degrees, beta = 104.58(2)degrees, gamma = 71.65(2)degrees, and Z = 2. Na-3[V(O-O)(4)]. 14H(2)O also crystallizes in the triclinic system, space group P (1) over bar, with a 8.657(2) Angstrom, b = 9.010(2) Angstrom, c = 7.520(2) Angstrom, alpha = 98.41(2)degrees, beta = 110.59(2)degrees, gamma = 63.99(2)degrees, and Z = 1. The peroxo O-O and the V-O-peroxo bond distances-are in the normal range. The V(O-O)(4)(3-) anions exhibit approximately dodecahedral symmetry with the peroxo ligands bound in an eta(2) fashion, It is unclear from the literature whether tri- or tetraperoxovanadate(V) should predominate under the experimental conditions used to obtain the crystals. Therefore, we have briefly reinvestigated by spectrophotometry their formation equilibria in neutral and basic solution, respectively. A value of K = 2 x 10(-6) has been determined at 25 degrees C in 0.050 M MOPS buffer at pH 7 for the reaction VO(O-O)(2)(-) + H2O2 = V(OH)(O-O)(3)(2-) + H+. The equilibrium constant for the reaction V(OH)(O-O)(3)(2-) + HO2- V(O-O)(4)(3-) + H2O was determined to be 0.48 at 25 degrees C in H2O2, HO2- buffer solutions at I = 2.5 M (NaCl). These results are consistent with previous reports that utilized V-51 NMR (Inorg. Chem. 1991, 30, 3718) and spectrophotometry (Bull. Sec. Chim. Fr. 1960, 819), respectively. In our hands, the syntheses reported to yield the blue solids Na[V(O-O)(3)] (Inorg. Chem. 1985, 24, 2706) and Na-2[V(O-O)(3)F] (Inorg. Chem. 1982, 21, 4020) afforded instead blue solids that appear to be impure samples of Na-3[V(O-O)(4)].