CTBN-modified epoxy resins (CMEs) with an interpenetrating-network (IPN) structure and a nanometer-sized morphology were prepared. Two systems of CMEs, called CNE/DDS/I-CTBN-B and CNE/DDS/I-CTBN-D, with IPN structures, were synthesized by heat-curing a homogeneous resin, CNE/DDS/CTBN/2-MI, obtained by mixing a carboxyl-terminated butadiene-acrylonitrile liquid rubber (CTBN) with a solution of polyglycidyl ether of o-cresol-formaldehyde novolac (CNE), 4,4'-diamino diphenyl sulfone (DDS), and 2-methyl imidazole (2-MI), in the presence of benzoyl peroxide and dicumyl peroxide, respectively. The IPN morphologies of the two systems of CMEs were identified by small-angle X-ray scattering by measuring the value of the specific interfacial surface area S-sp between the cured CNE/DDS matrix and the vulcanized CTBN. Properties such as fracture toughness, internal stress, and thermal and dynamic mechanical properties of these IPN-structured CMEs were studied in detail, and were compared with those of a conventional CME, CNE/ DDS/CTBN, obtained by dispersing CTBN particles in a crosslinked CNE/DDS matrix. (C) 2007 Wiley Periodicals, Inc.