Analytical studies have found an enrichment of the lighter Mo isotopes in oxic marine sediments compared to seawater, with isotope fractionation factors of -1.7 to -2.0 parts per thousand for Delta(MOsediment-seawater)-M-97/95. These data place constraints on the possible identities of dissolved and adsorbed species because the equilibrium isotope fractionation depends on the energy differences between the isotopomers of the adsorbed species, minor dissolved species, and the dominant solution species, MoO42-. Adsorption likely involves molybdic acid, whose structure is indicated by previous studies to be MoO3(H2O)(3). Here we used DFT calculations of vibrational frequencies to determine the isotope fractionation factors versus MoO42-. The results indicate that isotope equilibration Of MoO42- with MoO3(H2O)(3), yielding Delta(97/95) MOmolybdic acid-molybdate = 1.33 parts per thousand, is most likely responsible for the isotope fractionation of Mo between oxic sediments and seawater. The difference between the calculated value of A(97/95) MOmolybdic acid-molybdate for MoO3(H2O)(3) and the value observed in natural sediments and experiments is probably due to effects of solvation and adsorption onto the manganese oxyhydroxide surface.