Liquid rubbers that are ordinarily immiscible in liquid thermoset resins can be made miscible by the addition of at least one non-functional aromatic end-group to the polymer chains of such liquid rubber compositions. Such addition may be initiated by high-temperature peroxides in non-functional aromatic solvents. The liquid rubbers are found to improve the fracture toughness of cured thermoset resins while maintaining dimensional and heat resistance.