An equation of state is established for the gas phase Of CO2-H2O in the range 0-28 MPa and 323-645 K. The equation for gaseous CO2-H2O mixtures can accurately reproduce the experimental volumes with an average deviation of 0.25% and a maximum deviation of 2.8%. An accurate model for the molar volumes and densities of liquid CO2-H2O and CO2-H2O-NaCl mixtures is developed. The most accurate experimental density data for the CO2-H2O system in the range 273-623 K and 0.7-35 MPa can be reproduced within +/- 0.05%, and the average deviation is 0.008%. The model for the liquid CO2-H2O-NaCl mixtures is developed on the basis of our CO2-H2O model and the H2O-NaCl model of Rogers and Pitzer (J. Phys. Chem. Ref. Data 1982,11 (1), 15-81). No additional empirical parameter is introduced for the ternary. This model can predict the ternary density data within experimental errors and is expected to be valid up to 573 K. It is found that both the density model and the equation of state can be extrapolated up to 100 MPa or higher pressure with accuracy close to those of experiments. Computer programs for the related calculations can be downloaded from http://www.geochemmodel.org/programs.htm.