This paper considers the calculation of homogeneous chemical equilibria in high-pressure gaseous systems. A modified Redlich-Kwong-Soave equation of state, which represents satisfactorily single-phase fugacity coefficients of pure fluids, is extended to mixtures. Mixture parameters are derived from the corresponding pure-component ones by means of the classical mixing rules. Equilibrium compositions are predicted for the ammonia synthesis, the water-gas shift reaction, and the methanol synthesis. In the case of ammonia, extensive experimental data between 10 and 80 MPa are reproduced with good accuracy; when the model is extrapolated up to 350 MPa, still satisfactory results can be calculated. For the water-gas shift reaction, acceptable results are obtained with limited experimental data sets. As to methanol synthesis, an interesting behavior is evidenced when operating near methanol’s critical temperature. The proposed model is shown to be superior to both the Lewis-Randall approach and the original Redlich-Kwong-Soave equation of state and can be used to predict equilibrium conversion of gas-phase reactions under high pressures.