A new equation of state/excess Gibbs energy model, LCVM, has been developed. This work provides a theoretical justification for the mixing rule used in the LCVM model for the parameter alpha(= a/bRT, where a and b are cubic equation of state parameters). This is accomplished by determining standard states for pure components in a mixture, as well as a reference pressure, where the excess Gibbs energy expression obtained from the equation of state and the excess Gibbs energy model used give equal values.