Accurate assessment of the long-term security of geologic carbon sequestration requires knowledge of the mobility of carbon dioxide in brines under pressure and temperature conditions that prevail in subsurface aquifers. Here, we report Raman spectroscopic measurements of the rate of CO₂ diffusion in water and brines as a function of pressure, salinity, and concentration of CO₂. In pure water at 50 ± 2 °C and 90 ± 2 bar, we find the diffusion coefficient, D, to be (3.08 ± 0.03) × 10^-9 m²/s, a value that is consistent with a recent microfluidic study but lower than earlier PVT measurements. Under reservoir conditions, salinity affects the mobility of CO₂ significantly and D decreased by 45% for a 4 M solution of NaCl. We find significant differences of diffusivity of CO₂ in brines (0-4 M NaCl), in both the absolute values and the trend compared to the Stokes-Einstein prediction under our experimental conditions. We observe that D decreases significantly at the high CO₂ concentrations expected in subsurface aquifers (∼15% reduction at 0.55 mol/kg of CO₂) and provides an empirical correction to the commonly reported D values that assume a tracer concentration dependence on diffusivity.