Abstract Thermo-optical analysis of solution cast mixtures of poly (ethylene oxide) (POE) and poly (methyl methacrylate) (PMMA) has been carried out. Melting point depression was observed for increasing proportion of PMMA in the mixture. An analytical expression appropriate to the crystallineamorphous polymer pair has been applied to explain the melting point depression in terms of thermodynamic quantities. From this expression, the interaction parameter for the mixture has been evaluated. The influence of M w of POE on the binary interaction parameter has been studied.