This study reports new density measurements of the (CO2 + CO) system at temperatures from (283 to 373) K and pressures up to 48 MPa for four different mixtures, with compositions ranging from (5 to 50) mol% CO. A commercial vibrating-tube densimeter was used to measure the density of each mixture as a function of pressure and temperature. Temperature and pressure were measured with expanded uncertainties (k = 2) of 0.05 K and 0.035 MPa, respectively. The relative combined expanded uncertainty (k = 2) of the density was estimated to be between (0.2 and 1.8) %, with values ≤1% for most state points. The new data significantly expand the pressure and composition ranges of the available density data for the (CO2 + CO) system. Together with recently published vapour-liquid-equilibrium data, the new data enabled the development of an improved Helmholtz-energy-explicit mixture model. The new model is based on the mathematical approach of the GERG-2008 and EOS-CG models with new adjustable parameters. As a result, the new mixture model allows for a significantly more accurate description of the thermodynamic properties of the (CO2 + CO) system than GERG-2008 and EOS-CG. A detailed comparison among our density data, experimental data from the literature and the different mixture models is presented.