A tetrafunctional, anhydride hardened epoxy resin (EP) was modified with polyethersulfones (PESU) of different molecular mass (MM) without using additional solvent in 10 wt %. The morphology, thermal, and fracture mechanical properties of the EP/PESU systems were determined. PESU was present as dispersed phase in the EP matrix. The final morphology of EP/PESU depended on MM of PESU. PESU of low and medium MM formed submicron scale spherical droplets, whereas PESU of high MM was present in micron scale inclusions of complex (sea-island) structure. With increasing MM of PESU, the fracture mechanical properties, and especially the fracture energy, were increased. This was traced to differences in the morphology (dispersion of PESU) of the EP/PESU systems triggering different failure mechanisms. The onset of thermal degradation was slightly reduced, whereas the char yield enhanced by PESU modification compared with the reference EP. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 1193-1200, 2012