A new molar excess Gibbs energy (GE) expression for the correlation of vapor-liquid phase behavior for polymer solutions was developed from the Gibbs-Helmholtz relation

and a new activity model for polymer solutions based on the GE expression was then derived by accounting for the free volume effect.The model includes a combinatorial part

a residual part and also a free volume part.The calculated results by the present method were compared with those from the EFV

UFV and UNIFAC models.The average absolute deviation of solvent activities in 14 polymer solutions was 8.41% for EFV

7.74% for UFV

19.05% for UNIFAC and 3.52% for the present model.It is shown that the present model yields improved results over the other models and is able to describe the phase behavior for some polymer solutions over a wide range of concentration.