In order to design safe and effective storage of anthropological CO2 in deep saline aquifers, it is necessary to know the thermophysical properties of brine–CO2 solutions. In particular, density and viscosity are important in controlling convective flows of the CO2-rich brine. In this work, we have studied the effect of dissolved CO2 on the density and viscosity of NaCl and CaCl2 brines over a wide range of temperatures from 298 to 449 K, with pressures up to 100 MPa, and salinities up to 1 mol·kg–1. Additional density measurements were also made for both NaCl and CaCl2 brines with dissolved CO2 at salt molalities of 2.5 mol·kg–1 in the same temperature and pressure ranges. The viscosity was measured by means of a vibrating-wire viscometer, while the density was measured with a vibrating U-tube densimeter. To facilitate the present study, the theory of the vibrating-wire viscometer has been extended to account for the electrical conductivity of the fluid, thereby expanding the use of this technique to a whole new class of conductive fluids. Relative uncertainties were 0.07% for density and 3% for viscosity at 95% confidence. The results of the measurements show that both density and viscosity increase as a result of CO2 dissolution, confirming the expectation that CO2-rich brine solutions will sink in an aquifer. We also find that the effect of dissolved CO2 on both properties is sensibly independent of salt type and molality.