Phase behaviors of polydisperse polystyrene (PS)/nematic liquid-crystal systems [P-ethoxy-benzyhdene-p-n-butylaniline (EBBA)] are investigated with a thermo-optical analysis technique. We also develop a thermodynamic framework to describe the phase behaviors of polydisperse PS/EBBA systems. The proposed model is based on a modified double-lattice model to describe isotropic mixing and Maier-Saupe theory for anisotropic ordering. To correlate the polymer chain length and energy parameters in a nematic-isotropic biphasic region and to apply the primary interaction parameter in an isotropic-isotropic phase-transition behaviors of polydisperse PS/EBBA systems. The proposed model shows remarkable agreement with experimental data for the model systems in comparison with an existing model. (c) 2006 Wiley Periodicals, Inc.