This article features a simple method for describing the phase equilibrium of the ternary water–salt–carbon dioxide system. At first, an iterative solution is shown using well known equations of state and solubility correlations to predict the compositions of liquid and gas phases in a saline aquifer. By virtue of several assumptions, iterations can be avoided entirely in order to use the method for demanding large-scale numerical simulations. Subsequently, the iterative method as well as the simplification are evaluated against experimental data. Both solutions provide adequate accuracy for engineering problems such as subsurface carbon dioxide storage.