A new ionic liquid, tributyloctylphosphonium benzotriazolate ([P-4448][BTA]), was synthesized, and the density, viscosity, and electrical conductivity of [P-4448][BTA] were measured at atmospheric pressure. As temperature increased, the density and viscosity decreased, while the electrical conductivity increased. The density was fitted to a polynomial, and the transport properties were analyzed with the Vogel-Fulcher-Tammann equation. Moreover, the pressure-volume-temperature-composition (pVTx) relation for the CO2 + [P-4448]IBTA] binary system was investigated over the pressure up to 4 MPa at 298.15, 313.15, and 333.15 K [P-4448][BTA] showed the similar phase behavior as observed for typical physical absorbents: the CO2 solubility increased linearly at low pressures and the increment gradually decreased at high pressures. The Henry's constant and thermodynamic parameters of solution were derived from the pVTx data to discuss CO2 absorption in [P-4448][BTA]. The present experimental results were compared with those for the related ionic liquids, composed of [P-4448](+) and [BTA](-), as reported previously. (C) 2019 Elsevier B.V. All rights reserved.