Extractive distillation is a distillation technique that can be applied for separation of azeotropic mixtures and mixtures of close-boiling compounds. In extractive distillation a solvent is applied to increase the relative volatility by interacting with the components in the system, typically with a difference in affinity for the components. To study the relation between the affinity strength and the effect of the solvent on the relative volatility, the research here presented focused on a solvent selection for the separation of two close-boiling, highly polar binary systems. 1) The acids valeric acid and its isomer 2-methylbutyric acid (ΔTboil = 10°C, ΔpKa = 0.1), and 2) the bases diethyl methylamine and diisopropylether (ΔTboil = 3-6°C, ΔpKb ≈ 14). For both the acids and the bases the binary VLE-data and ternary data with solvents were measured using an ebulliometer. The solvents resulted in negative deviations of Raoult's law. The solvent selection was performed based on literature, followed by an experimental evaluation of the possible solvent types. For the valeric acid mixture the solvent 2-methylbutyrate, which is similar in structure to the components in the mixture, appeared to be most promising. In the ether-amine mixture a series of hydrogen bonding solvents with a hydroxyl group were found most promising, and also the regeneration of the solvent was an important factor in the study.