A local composition model based on the lattice theory, which is a modified nonrandom factor (NRF) model, is developed for the representation of the excess Gibbs energy of polymer solutions. The model consists of two contributions due to the configurational entropy of mixing, represented by the Flory-Huggins (FH) relation, and to the enthalpic contribution, represented by local compositions through norrandom factors. The model is valid for the whole range of polymer concentration, from dilute solution up to saturation. Results are compared with those obtained from the F-H theory, UNIQUAC, Chen's NRTL model and Wu et al. NRTL model. The model presented in this work consistently produces better results.