The P-V-T-x relationship of three systems, pure carbon dioxide (CO2) and two binaries CO2 + butane (C4H10) and CO2 + isobutane (iC(4)H(10)), were measured with a constant volume method. The experimental temperature was set to be 360.00 K, and the range of pressure was from 1.0 to 10.5 Wa. For the mixtures, the mole fraction of either C4H10 or iC(4)H(10) was prepared to be less than 20.10 mol%. The data for CO2 obtained in this study agreed well with the literature. Experimental data were correlated with the Benedict-Webb-Rubin (BWR) equation of state, where the three constants for CO2 were redetermined in our previous work. The calculation using BWR equation with the Stotler-Benedict type mixing rules could correlate the experimental data of the two binaries well. The partial molar volume and the number of solvating molecules for C4H10 and iC(4)H(10) in the mixtures were calculated from the BWR equation. (C) 2003 Elsevier B.V. All rights reserved.