The densities for {x(1)CO(2) + x(2)CH(4) + (1-x(1)-x(2))N-2} were measured at the pressures (19.94, 39.94, 59.93, 99.93) MPa and temperatures (323.15, 373.15, 473.15, 573.15) K using a custom-designed, high-pressure, high-temperature vibrating-tube densimeter. Excess molar volumes V-m(E) were calculated from the experimental results. Although V-m(E) is generally positive, negative deviations are observed in ternary mixtures with compositions close to those of binary mixtures that exhibit negative deviations. V-m(E) generally decreases as pressure increases from 20 MPa to 100 MPa. V-m(E) is typically less than +/- 4 per cent of the total volume of the mixture. V-m(E) increases substantially as (p, T) conditions approach the critical point of CO2. Predictions of V-m(E) for ternary mixtures may be made from experimental results for the binary subsystems. Comparisons with experimental results indicate that these methods are reasonably accurate for estimating the volumetric properties of ternary gas mixtures.