The previous theory of pure polymer fluid is extended to polymer solutions. The partition function and the complete thermodynamic description for polymer solutions on a lattice model are obtained. With neglecting of the influence of preferential interaction on configuration, the free energy of Flory-Huggins is recovered. The temperature-independent portion of effective interaction parameter chi is explained from the molecular origins, and the observed composition dependence of chi is provided on a molecular basis. The values calculated from the present theory match the experimental spinodals data. However, the mean field approximation has an important effect not only on spinodals and binodals, but also on the interaction parameter. (C) 2003 American Institute of Physics.