The morphology-toughness relationship of vinyl ester resins was studied as a function of their modification. Bisphenol A based and novolac-based vinyl ester resins were modified by a star-shaped polyether polymer with vinyl and hydroxyl functionalities and/or by a polyisocyanate. The polyisocyanate-containing systems were termed vinyl ester/urethane hybrids. The morphology of the crosslinked resins was studied with dynamic mechanical thermal analysis and atomic force microscopy with ion-eroded specimens and discussed. The toughness of the crosslinked resins was assessed by the linear elastic fracture mechanics with compact tension specimens. The fracture toughness and energy changed fairly linearly as functions of M, and M-c(0.5), respectively, where M, is the mean molecular mass between crosslinks. (c) 2006 Wiley Periodicals, Inc.