A two-sinker magnetic suspension densimeter was used to carry out accurate density measurements-on binary mixtures (argon + carbon dioxide) with carbon dioxide mole fractions of 0.50000, 0.49975, and 0.75093. The measurements cover a temperature range from T = (273.15 to 323.15) K with pressures up to 9.1 MPa or the dew-point pressure, whichever was lower. The mixtures were prepared gravimetrically. With all measurement uncertainties in temperature, pressure, density, and composition taken into account, the relative combined expanded uncertainty (k = 2) in density was estimated to be less or equal to 0.033%. Relative deviations of the experimental densities from the GERG-2008 equation of state were within 0.31% for the mixtures with 0.50000 and 0.49975 mole fraction carbon dioxide and up to 1.90% for-the mixture with 0.75093 mole fraction carbon dioxide. In contrast, the relative deviations from a recently developed multiparameter equation of state (EOS) optimized for combustion gases (EOS-CG) were less than 0.30% for the three mixtures under study. Third-order virial equations were fitted to the measured density values. The correlated molar masses at different temperatures agreed within less than 0.01% with the molar masses determined from the gravimetric mixture preparation. Values and uncertainties of the second and third virial coefficients as well as of the second interaction virial coefficient were calculated.