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To develop an equation of state with simple structure and reasonable accuracy for engineering application, Redlich-Kwong equation of state was modified for gaseous CO₂ and CO₂-H₂O mixtures. In the new modification, parameter a of gaseous CO₂ was regressed as a function of temperature and pressure from recent reliable experimental data in the range: 220-750 K and 0.1-400 MPa. Moreover, a new mixing rule was proposed for gaseous CO₂-H₂O mixtures. To verify the accuracy of the new modification, densities were calculated and compared with experimental data. The average error is 1.68% for gaseous CO₂ and 0.93% for gaseous mixtures of CO₂ and H₂O. Other thermodynamic properties, such as enthalpy and heat capacities of CO₂ and excess enthalpy of gaseous CO₂-H₂O mixtures, were also calculated; results fit experimental data well, except for the critical region.