In an effort to improve toughness N,N,N’,N’-tetraglycidyl-4,4’diaminodiphenyl methane (TGMDA) was modified with poly(arylene ether-co-imidazole)s (PAE-co-Is). The PAE-co-Is contain imidazole units distributed randomly along the backbone which react with the epoxies upon curing. A system modified with 10% w/w high molecular weight polymer exhibited increases in fracture toughness of 1.7 times that of the unmodified system; however, controlled molecular weight PAE-co-Is were required to retain processablility. Modification also resulted in significant decreases in the tetrahydrofuran sensitivity. Scanning electron photomicrographs of the fractured surface showed stress whitening with no clear phase separation whereas poly(arylene ether)s without imidazole groups exhibited gross phase separation.