The structure of the heteropoly 18-molybdopyrophosphate anion [(P2O7)Mo18O54](4-) (I), originally synthesized by Himeno et al. (Bull. Chem. Sec. Jpn. 1990, 63, 1602), has been determined. The tetra-n-butylammonium salt of I crystallizes in the orthorhombic space group Pnn2 with a = 26.760(2) Angstrom, b = 28.632(3) Angstrom, c = 15.831(2) A, V = 12130(2) Angstrom(3), and Z = 4. Structural analysis based on 6525 independent reflections leads to a solution, R = 7.69% (I > 2 sigma(I)). The anion has virtual D-3h symmetry With a structure based on fusion of two B-type PMo9O34 moieties. Consequently the enclosed pyrophosphate anion is constrained to have a linear P-O-P bond (177.7(13)degrees) and an eclipsed conformation. As for other molybdophosphates, the Mo-O(P) bonds are long, corresponding to bond orders of 0.1-0.25. The green one-electron-reduced species [(P2O7)Mo18O54](5-) (I’) and the blue two-electron-reduced species [(P2O7)Mo18O54](6-) (I") show, in acetonitrile solution, a single P-31 NMR resonance at -22.8 ppm (Delta nu(1/2) 8.0 Hz) and -23.1 ppm (Delta nu(1/2) 2.1 Hz), respectively. The ESR spectrum of a frozen solution (77 K) of I’ is a barely resolved axial pattern with g(parallel to) = 1.92 and g(perpendicular to) = 1.94. Comparisons are made with reduced Keggin, [(PO4)Mo12O36](4-), and Dawson, [(PO4)(2)Mo18O54](7-), anions.