The solvent transport number, Δ, of dimethyl sulfoxide and Gibbs solvation energies of silver acetate in the binary solvent systems, water-DMSO and methanol-DMSO, were determined by employing EMF and solubility measurements. While the transfer free energy of the salt increases from water to water-DMSO mixtures (up to XDMSO=0.7) and then decreases, it continuously decreases from methanol to methanol-DMSO mixtures. In both mixed solvents, ΔG°t(Ag+) decreases down to pure DMSO and that of acetate ion increases with increasing composition of DMSO indicating that silver ion is preferentially solvated by DMSO and acetate ion by water or methanol in these mixtures. The solvent transport numbers, Δ, of DMSO are positive throughout, passing through a maximum at XDMSO=0.45 (Δ=1.0) in the case of water-DMSO mixtures and at XDMSO=0.25 (Δ=1.8) in methanol-DMSO mixtures. This observation is shown to be in accord with the conclusions arrived at from the transfer energy data of the salt in the two mixtures. © 1982 Springer-Verlag.