The phase separation process of an epoxy prepolymer based on diglycidyl ether of bisphenol A (DGEBA) with a thermoplastic polystyrene (PS) was thermodynamically studied in the frame of the Flory-Huggins theory. The thermodynamic treatment was carried out in two steps: first analysing the phase separation in cloud point conditions, and second analysing the advance of the phase separation for two compositions of 2 and 10% in volume of PS. The effect of the polydispersity of thermoplastic on phase separation was also studied. The polydispersity of PS produces a displacement of the threshold temperature to lower thermoplastic volume fraction (between 2 and 3%) and higher temperature value and the fact that the shadow curve and coexistence curves do not superimpose with the cloud point curve. Theoretical calculations of molecular weight distributions of PS at different degrees of phase separation were realized and different average molecular properties were obtained in each separated phase. (c) 2005 Elsevier Ltd. All rights reserved.