The (p, rho, T, x) properties for binary mixtures of CO2 (volume fraction purity 0.99999) and isobutane (mole fraction purity 0.99988) {x(1) CO2 + x(2) isobutane (x(1) = 0.2482, 0.4718, and 0.7506)} were measured in the compressed liquid phase using a metal-bellows variable volumometer. Measurements were conducted from T = (280 to 440) K and (3 to 200) MPa. The expanded uncertainties (k = 2) were estimated to be: temperature, <3 mK; pressure, 1.5 kPa (p <= 7 MPa), 0.06%(7 MPa < p <= 50 MPa), 0.1%(50 MPa < p <= 150 MPa), 0.2% (p > 150 MPa); density, 0.10%; and composition, 4.4 10(-4). At >100 MPa and T= (280 or 440) K, the uncertainties in the density measurements increased to 0.14% and 0.22%, respectively. The data are compared with the available equation of state. The excess molar volumes, v(m)(E), of the mixtures were calculated and plotted as a function of temperature and pressure. (C) 2011 Elsevier Ltd. All rights reserved.