Thermosetting resins are provided that are toughened with an irradiated thermoplastic toughening agent. The resins have reduced levels of solvent-induced micro crack formation and do not lose their adhesiveness when attacked by solvent. The thermoplastic toughening agent is treated with a sufficient amount of high-energy radiation (e.g. electron beam or gamma rays) to cause reductions in solvent-induced micro crack formation and solvent-induced loss of adhesiveness when compared to the same toughened thermosetting resin in which a non- irradiated version of the thermoplastic toughening agent is used.