Phase diagrams of tertiary blends of poly(phenylene ether) (PPE, M-n = 1.2 and 12 kg mol(-1)), polystyrene (PS, M-n = 22.5 kg mol(-1)), and diglycidyl ether of bisphenol A (DGEBA) were experimentally obtained in an extended range of temperatures and fitted with the Flory-Huggins model using three binary interaction parameters. A significant increase in miscibility together with the appearance of an immiscibility loop was found for PPEs with M-n values comprised in the range between 1 and 10 kg mol(-1). This enables us to obtain initial homogeneous Solutions in regions of high DGEBA concentrations, a possibility that was not previously reported for this tertiary blend. This opens new possibilities for the toughening of epoxies replacing a single thermoplastic with a thermoplastic blend where both components (PS and PPE) are completely miscible. (c) 2006 Wiley Periodicals, Inc.