Single droplet mass transfer of acetic acid from aqueous phase into triisooctylamine-based solvents modified with the Lewis basic modifier tributyl phosphate and the diluent Shellsol T was investigated for applicability in solvent design. Solvent composition of triisooctylamine-based solvent mixtures has a decisive effect on liquid-liquid partition of acetic acid. The modifier is needed to correct acid association of the ammonium salt, while the diluent mainly affects the viscosity, the density, and the interfacial tension of the solvent. The results show that single droplet mass transfer is controlled by solvent viscosity.