The EPR spectroscopic parameters of a series of Mo(V) oxyhalide anions, [MoOX(5-n),(H2O)(n)]((2-n)-) (X = F, Br, n = 0; X = Cl, n = 1), were obtained in fluid solutions and frozen glasses or (X = Cl, Br) doped into single crystals of a diamagnetic host lattice. The electronic structures of the complexes were approximated by optimizing the electronic structural parameters of a LCAO model to reproduce the experimentally observed EPR parameters. The results indicate quantitatively that the most important contribution to deviations from g = g(e) in the complexes is metal-ligand covalency. Charge transfer excited state mixing and ligand spin-orbit coupling (for X = Cl, Br) provide significant but smaller contributions to the EPR parameters. The isotropic molybdenum hyperfine coupling constants are also shown to be dominated by Fermi contact interactions. A number of implications with respect to the EPR spectroscopy of molybdenum oxidoreductases are noted. The crystal and molecular structure of a diamagnetic Nb(V) lattice is also reported. (H(2)dafone)[NbOCl4(H2O)]Cl (dafone = 4,5-diazafluoren-9-one) crystallizes in the monoclinic space group C2/c with a = 16.0043(2) Angstrom, b = 24.8021(3) Angstrom, c = 10.0162(2) Angstrom,beta = 121.048(1)degrees, and Z = 8.