AbstractCompatibility or miscibility of polyethersulfones (ICI: Victrex 100P and 300P) and a tetrafunctional epoxy (Ciba‐Geigy: MY‐720), cured with an aromatic anhydride, has been studied using scanning electron microscopy, x‐ray microanalysis, and dynamic mechanical spectroscopy. Fracture toughness of epoxy and blends of an epoxy and polyethersulfones has been measured using three‐point bend tests (ASTM: E‐399–81), and the energy release rate (GIC) for the three materials has been compared as a function of test temperature. Fracture surfaces were examined by x‐ray microanalysis for detecting concentration of sulfur, present in polyethersulfones, in the matrix and precipitated phase. The influence of morphology of epoxy/polyethersulfone blends on its fracture toughness and toughening mechanism has been studied. A toughening criterio