Phase behaviours of polydisperse polystyrene (PS)/nematic liquid crystals (LCs) P-ethoxy-benzylidene-p-n-butylaniline (EBBA) are investigated by thermo-optical analysis (TOA) technique. In this study, to describe nematic-isotropic transition of the polydisperse PS/EBBA systems, a continuous distribution function to obtain the composition of polydisperse polymers is considered precisely. We apply these molar mass distributions to the extended Flory-Huggins model. The proposed semi-empirical model gives a remarkable agreement with experimental data for the model systems. (c) 2006 Elsevier B.V. All rights reserved.