Densities rho of pure CO2, CH4, and {(1-x)CO2 + xCH(4)}, {(1-x)CO2+ xN(2)}, and {(1 - x)CH4 + xN(2)} were measured at the pressures (9.94, 19.94, 29.94, 39.94, 59.93, 79.93, and 99.93) MPa and temperatures (323.15, 373.15, 473.15, and 573.15) K. For each binary system, results were obtained at intervals of 0.1 . from mole fraction x = 0 to x = 1. The results were obtained with a custom-designed, high-pressure, high-temperature vibrating-tube densimeter. Excess molar volumes V-m(E) were calculated from the densities for the end members and mixtures. V-m(E) is generally positive, although negative values of V-m(E) were observed for some CO2-bearing mixtures. V-m(E) is typically less than +/- 4 per cent of the total volume of the mixture. A dramatic increase in V-m(E) is observed for CO2-containing mixtures as (p, T) conditions approach those of the critical point for CO2. The values of V-m(E) are accurately represented by two-parameter Margules equations. Our results are in good agreement with those obtained by previous investigators.