The tungstorhenate(V) heteropolyanion [W9ReO32](5-) has been isolated as guanidinium and cesium salts from reaction of [ReO2(PPh3)(py)(3)](+) with sodiom tungstate. Crystallographic analysis of black Cs-5[W9ReO32]. 3H(2)O [triclinic, P1 or P1; a = 10.194(1), b = 11.503(2), c = 9.682(1) Angstrom : alpha = 100.55(1), beta = 115.81(1), gamma = 99.13(1)degrees; Z = 1], based on 3743 reflections, shows the anion to be isostructural with decatungstate, [W10O32](4-). Refinement in P1 led to reliability indices R = 0.084, R-W = 0.046. Electrochemical investigation revealed the existence of Re-VI and Re-VII analogues, which were hydrolytically unstable in aqneous solution but which were isolated as crystalline tetra-n-butylammonium and tetra-n-heptylammonium salts, respectively, from nonaqueous solvents. The tetra-n-butylammonium salts of [(W9ReO32)-O-VI](4-) and [W10O32](4-) were shown to be isomorphous by X-ray powder diffraction. Simulation of the Q-band ESR spectrum of [(W9ReO32)-O-VI](4-) (polycrystaline solid solution in [W10O32](4-)) gave g(x) = 1.69(1), g(y) = 1.730(2) and 10(4)A(x)(Re-185,Re-187, I = 5/2) = (-)252(10), 10(4)A(Y) = (-)398(10), 10(4)A(z) = (-)653(5) cm(-1). The orthorhombic ESR spectrum proves that the Re atom occupies one of the eight equivalent "equatorial" sites in the decatungstare structure.