The behavior of protonated binary solvents injected into deuterated binary mobile phases in capillary LC is studied with NMR. Specifically, the solvent elution is followed on-flow with a capillary LC coupled to a 900 nL volume microcoil NMR probe. A range of identical composition 5% protonated (and 95% deuterated) solvents is injected into composition-matched deuterated mobile phases of CD₃CN/D₂O and CD₃OD/D₂O. The protonated components separate for all solvent combinations except at 80% CD₃CN/20% D₂O and ∼72% CD₃OD/28% D₂O where only a single retention time is observed. The more hydrophilic protonated component, HOD, elutes first with higher percentages of hydrophilic solvent, D₂O, in the mobile phase whereas retention is reversed with the higher percentage of the more hydrophobic solvent (CD₃CN and CD₃OD) in the mobile phase. The hydrophilic/hydrophobic nature of the chromatographic system as a function of mobile phase composition is characterized by following the retention times of protonated solvents.