Ultrahigh efficiency separations based on the presence of one deuterium in benzene, toluene, and naphthalene were achieved by recycle chromatography using C18 silica columns. Larger isotopic separation factors, alpha(H/D), were observed in methanol-water than in acetonitrile-water, when the mobile phases provided similar retention factors (k), or similar methylene selectivity, alpha(CH2). Isotopic resolutions between nondeuterated and perdeuterated aromatic hydrocarbons at long separation times were estimated by using the plate counts obtainable by a recycle operation as a function of a cycle time, along with the retention factors and the separation factors experimentally observed. Methanol-acetonitrile-water ternary mobile phases were predicted to provide the greatest resolution per unit time, and actually enabled separation by the difference of one H/D substitution on the aromatic hydrocarbons with alpha(H/D) = 1.008 or less, and also the differentiation of the isomers of monodeuterated toluene with alpha(toluene-4-d/toluene-alpha-d) = 1.0016. The discrimination mechanism of H/D isotopic species is discussed based on the dispersion interactions of a CH/CD group of the solute with the stationary phase as well as the mobile phase.