CO₂ CH₄, and N₂ are key volatile constituents of the earth; therefore, thermochemical data on these gas species and their mixtures find application in geochemical modeling of crustal fluids in a wide variety of geologic environments. However, volumetric data for gaseous mixtures are scarce, particularly at high pressures and temperatures. In this study, the volumetric properties of binary and ternary mixtures in the system CO₂-CH₄-N₂ were measured at 100-1000 bars, 50-300°C, using a vibrating-tube densimeter constructed at the Oak Ridge National Laboratory. An isobaric, isothermal flow-through method was employed to obtain a statistically significant number of measurements for the period of vibration at each set of P-T-X conditions. During each experimental session, three well Characterized standard gases (He, N₂ and Ar) were used to calibrate the response of the instrument. Precisions achieved in experimentation were: P, ±0.1 bar; and T, ±0.01 °C. Conservative estimates of accuracy are: P, ±0.2 bar; and T, ±0.05 °C. The uncertainties associated with volumetric determinations derive from counting statistics and vary as a function of the density and composition of the gas mixture. Generally, the uncertainty in volume ranges from ±0.1 to ±0.3 %. / Ph. D.