Experiments modeling diffusion of Mo in aqueous solutions have been performed and, using multicollector ICP-MS, the ratios of the diffusivities of Mo isotopes, D97Mo/D95Mo, in aqueous solutions have been determined. Diffusion of MoO42- ions in solution was concomitant with Mo isotopic fractionation with D97Mo/D95Mo = 0.99988 ± 0.00004 (2σ for n = 3). In contrast, during diffusion of Mo polyanions, such as Mo7O246- and Mo8O264-, no measurable isotope fractionation has been found with D97Mo/D95Mo = 1.00000 ± 0.00002 (2σ for n = 3). These results indicate the need for due consideration to Mo speciation when attempting to interpret the role of diffusive fluxes in the formation of Mo isotopic signatures in nature. They also raise the possibility that the various chemical forms of other transition metals may be characterized by species-specific isotopic fractionation effects during physicochemical reactions.