The calculation of solubility in supercritical fluids requires models of high precision in the near-critical region, and this is not always the case with classical models. The model proposed here is based on the relations which exist between the Helmholtz energy of pure components and the excess Helmholtz energy in a mixing process at constant packing fraction. The pure component properties are represented using any equation of state which is actually included in the equation of state of the mixture. The expression of van Laar was used for the residual term of the excess function. This model was applied to the solid-fluid equilibria occurring in the binary systems composed of a hydrocarbon solid and supercritical carbon dioxide. The best results in the calculations of solubility and molar volumes of the mixture were obtained by using precise equations of state for each compound.