A modified perturbed hard-sphere equation of state, represented by the Carnahan-Starling equation with the van der Waals attractive part, has been investigated using the Mollerup approach for the energy parameter a for a pure substances. The equation has been extended to mixtures, for both vapor-liquid equilibria and solid-supercritical fluid equilibria, using the Boublik-Mansoori equation with the same van der Waals attractive part. The vapor pressure, density, second virial coefficient, specific heat capacity and enthalpy were examined for pure substances. The new equation was also able to predict accurately the vapor pressures of fatty acids. The predictions of both vapor-liquid equilibria and solid-supercritical fluid equilibria, using the one fluid van der Waals mixing rules with one binary interaction parameter, are significantly better than those obtained by cubic equatins of state, especially for large molecules. The predictions for solid-supercritical fluid systems were improved by incorporating solute-solute interaction parameters. Density dependent mixing rules were also examined.