A mixed-valence vanadium phosphate, NH4[(V2O3)(2)(4,4'-bpy)(2)(H2PO4)(PO4)(2)](.)0.5H(2)O, has been synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction. It crystallizes in the monoclinic space group C2/c (No. 15) with a = 12.6354(8) Angstrom, b = 9.9786(6) Angstrom, c = 23.369(1) Angstrom, beta = 92.713(1)degrees, and Z = 4 with R-1 = 0.0389. The structure consists of dimers of edge-sharing vanadium(IV,V) octahedra that are connected by corner-sharing phosphate tetrahedra to form layers in the ab-plane, which are further linked through 4,4'-bipyridine pillars to generate a 3-D framework. Magnetic susceptibility confirms the valence of the vanadium atoms. The P-31 MAS NMR spectrum shows a resonance centered at 80 ppm with a shoulder at ca. 83 ppm in an intensity ratio close to 1:2, which correspond to two distinct P sites. The observed large downfield P-31 NMR shifts can be ascribed to magnetic exchange coupling involving phosphorus atoms. The unpaired electron spin density at the phosphorus nucleus was determined from variable-temperature P-31 NMR spectra. The H-1 MAS NMR spectrum was fitted to six components in accordance with the structure as determined from X-ray diffraction.