Using lattice-fluid model,a continuous thermodynamic framework is presented forphase-equilibrium calculations for binary solutions with a polydisperse polymer solute.A two-stepprocess is deslgned to form a real polymer solution containing a solvent and a polydisperse polymersolute occupying a volume at fixed temperature and pressure.In the first step,close-packed purecomponents including solvent and polymers with different molar masses or different chain lengths aremixed to form a closed-packed polymer solution.In the second step,the close-packed mixture,con-sidered to be a pseudo-pure substance is mixed with holes to form a real polymer solution with a vol-ume dependent on temperature and pressure.Revised Freed’s model developed previously is adoptedfor both steps.Besides pure-component parameters,a binary size parameter c_r and a binary energyparameter ε_(12) are used.They are all temperature dependent.The discrete-multicomponent approach isadopted to derive expressions for chemical potentials,spinoda Using lattice-fluid model, a continuous thermodynamic framework is presented forphase-equilibrium calculations for binary solutions with a polydisperse polymer solute. A two-stepprocess is deslgned to form a real polymer solution containing a solvent and a polydisperse polymersolute occupying a volume at fixed temperature and pressure. in the first step, close-packed purecomponents including solvent and polymers with different molar masses or different chain lengths are fixed to form a closed-packed polymer solution.In the second step, the close-packed mixture, con-sidered to be a pseudo-pure substance is mixed with holes to form a real polymer solution with a vol-ume dependent on temperature and pressure. Revised Freed’s model developed previously was for both steps. Besides pure-component parameters, a binary size parameter c_r and a binary energyparameter ε_ (12) are used. They are all temperature dependent. The discrete-multicomponent approach isadopted to derive expressions for chemical potentials, spinoda