A common method for toughening thermoset resins is the addition of butadiene-acrylonitrile based rubber modifiers. A difficulty experienced by diglycidyl ether of bisphenol-A (DGEBA) vinyl esters (VE) compared to other thermosets is the solubility of the butadieneacrylonitrile modifier. In this study, a miscible system was prepared by reducing the VE monomer molecular weight to 540 g/mol from industrially used monomers possessing molecular weights greater than 700 g/mol. A main objective of this study was to toughen the vinyl ester while limiting plasticization by varying rubber reactivity and concentration. All modified samples experienced a significant increase in fracture,toughness with some degree of plasticization, as shown by losses in modulus and glass transition temperature (T-g). Epoxy terminated rubber (ETBN) yielded a higher toughness than vinyl terminated rubber (VTBN) due to the difference in rubber particle formation and resulting, morphology. Chemical linkage of VTBN to the vinyl ester matrix hindered complete phase separation, but helped the retention of mechanical properties when compared to ETBN. (C) 2004 Published by Elsevier Ltd.