The (p, rho, T, x) properties for (x(1) n-butane + x(2) isobutane) with x(1) = (0.2625, 0.4913, and 0.7508) in the compressed liquid phase were measured by means of a metal-bellows variable volumometer over the temperature range from (280 to 440) K at pressures from (1 to 200) MPa. The mole fraction purities of the n-butane and isobutane used in the measurements were 0.9997 and 0.9999, respectively. The expanded uncertainties (k = 2) in temperature, pressure, density, and composition measurements have been estimated to be less than +/-3mK; 1.4 kPa (p <= 7 MPa), 0.06% (7 MPa < p <= 50 MPa), 0.1% (50 MPa < p <= 150 MPa), and 0.2% (p > 150 MPa); 0.09%; and 4.4 . 10(-4), respectively. In the region above 100 MPa at T = (280 and 440) K, the uncertainty in density measurements increase from 0.09% to 0.13%, and 0.22%, respectively. Comparisons of the available equation of state with the present measurements are reported. On the basis of the present results, the excess molar volume upsilon(E)(m) of the mixtures was calculated and illustrated as a function of temperature and pressure. (C) 2007 Elsevier Ltd. All rights reserved.