Liquid nitrile rubbers with vinyl (VTBN), carboxyl (CTBN) and epoxy (ETBN) and hyperbranched polyesters with vinyl (VHBP) and epoxy (EHBP) functionalities were added in 10 wt% to a bisphenol-A based vinylester-urethane hybrid resin (VEUH) for its toughening. The fracture energy (G,) was determined on compact tensile specimens at ambient temperature. High toughness improvement was achieved by adding ETBN and CTBN to VEUH. It was established that a change in the initial stoichiometry of OH/NCO may affect G(c). The combination of CTBN with other additives in 1: 1 ratio yielded a synergistic effect with respect to G(c). Changes in G(c) were explained by differences in the fracture mode based on fractographic inspection of the fracture surface of the specimens. It was shown the same G(c) may be derived from completely different failure scenarios.