The limiting activity coefficients (gamma(infinity)(13)) for 65 solutes and water in the tetrabutylphosphonium bis{(tri-fluomethyl)sulfonyl}imide ionic liquid (IL), [P-4,P-4,P-4,P-4][NTf2] were determined by gas-liquid chromatography at six temperatures in range of (358.15-408.15) K. The DSC data are presented for new, synthesized by us [P-4,P-4,P-4,P-4][NTf2]. The thermodynamic functions, the limiting partial molar excess Gibbs energy, enthalpy and entropy were calculated. The values of selectivity and capacity for wateributan-1-ol separation problem were calculated from In and compared to literature values for phosphonium-based or other selected ionic liquids for the same separation problem. On the bases of the results the intermolecular interactions of the chosen IL and solutes was discussed. The data presented here shows that [P-4,P-4,P-4,P-4][NTf2] reveals second large ever published selectivity for the water/butan-1-ol separation. (C) 2018 Elsevier B.V. All rights reserved.