A microwave re-entrant cavity resonator was used for the determination of dew points, dielectric permittivities, and molar densities of two gravimetrically prepared (argon + carbon dioxide) mixtures, with carbon dioxide mole fractions of 0.9495 and 0.7509. Isochoric dew point measurements of a (0.0505 argon + 0.9495 carbon dioxide) mixture were carried out over the temperature range from (257 to 291) K at pressures between (2.4 and 6.0) MPa. The measured dew-point pressures were consistent with the predictions of the recently developed multiparameter equation of state optimized for combustion gases (EOS-CG) within 0.35 %. Measurements of each mixture's dielectric permittivity in the single-phase gas region over the temperature range from (273.2 to 313.3) K at pressures up to 6.5 MPa were used to determine the mixture molar densities at the same conditions. The method used to determine mixture molar densities from microwave-cavity measurements is based on an inversion of the polarizability mixing rule developed by Harvey and Prausnitz. The microwave-determined mixture densities had relative deviations from values measured for the same mixture with a two-sinker magnetic-suspension densimeter of 0.3 % or less. The new mixture density data agree within 0.37 % of predictions made using the EOS-CG and help identify literature data sets that should receive lower weighting in future model development.