Thermosetting resins were synthesized by the partial replacement of the synthetic epoxy prepolymer based on diglycidyl ether of bisphenol A (DGEBA) with increasing amounts of epoxidized soybean oil (ESO) with methyltetrahydrophthalic anhydride as a crosslinking agent and 1-methyl imidazole as an initiator. Calorimetric studies showed a drop in the reaction heat with ESO content; this was associated with the lower reactivity of oxirane rings in ESO due to steric constrains. The effects of the replacement of increasing amounts of synthetic DGEBA with ESO on the network properties, such as the storage modulus (E') in the glassy and rubbery regions, glass-transition temperature (T(g)), and impact and compressive properties were examined. All formulations were transparent, although phase-separated morphologies were evidenced by scanning electron microscopy observations. The intensity of the transmitted light passed to a minimum at a short reaction time associated with the cloud point and then increased continuously until the refractive index of the dispersed phase approximated that of the continuous phase at complete conversion. The combination of DGEBA with 40 wt % ESO resulted in a resin with an optimum set of properties; E' in the glassy state was 93% of that of the neat DGEBA resin, T(g) decreased only about 11 degrees C, and the impact strength increased about 38% without a loss of transparency. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 789-798, 2011