As a thermodynamic property, the density of CO2 mixtures plays an important role on carbon capture and storage. The densities of two CO2/N-2/O-2 ternary mixtures were measured at temperatures from (300.15-353.15) K and pressures from (5-18) MPa using a high-precision magnetic suspension balance. The temperature and pressure uncertainties were 0.01 K and 0.005 MPa, respectively. The density measurement uncertainty was estimated as 0.138 kg.m(-3). Impurities decreased the density of pure CO2 and the more the impurities were, the larger the reduction was. The densities of mixtures were calculated using GERG-2008 EOS and compared with experimental data. When the temperature and pressure were far away from the critical region, the discrepancy between the experimental data and GERG-2008 EOS calculations was small.