Thermosetting resins are provided that are toughened with an irradiated thermoplastic toughening agent and which have reduced levels of solvent-induced micro crack formation. The thermoplastic toughening agent is treated with a sufficient amount of high-energy radiation (e.g. electron beam or gamma rays) to cause a reduction in solvent-induced micro crack formation in the cured resin when compared to the same toughened thermosetting resin in which the non-irradiated version of the thermoplastic toughening agent is used.