An intercrosslinked network of epoxy matrix systems modified by cyanate ester (CE) and bismaleimide (BMI) was developed. Epoxy systems modified with 4%,8%, and 12% (by weight) cyanate ester were made by using epoxy resin and cyanate ester, with diaminodiphenylmethane as the curing agent. The reaction between the cyanate ester and the epoxy resin during the cure process of cyanate ester-modified epoxy systems was studied using Fourier transform infrared spectroscopy. The cyanate ester-toughened epoxy systems were further modified with 4%, 8%, and 12% (by weight) bismaleimicle (N,N'-bismaleimido-4,4'-di-phenylmethane). BMI-CE-Epoxy matrices were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis, and heat deflection temperature analysis. The matrices, in the form of castings, were characterized for their mechanical properties such as tensile strength, flexural strength, and unnotched Izod impact test as per ASTM methods. Mechanical studies indicated that the introduction of cyanate ester into epoxy resin improved the toughness and flexural strength, with a reduction in tensile strength and glass-transition temperature, whereas the incorporation of bismaleimide into epoxy resin influenced the mechanical and thermal properties according to its percentage content. However, the introduction of both cyanate ester and bismaleimide influenced the mechanical properties according to their percentage content. DSC thermograms of cyanate ester-modified epoxy and BMI-modified epoxy showed unimodal reaction exotherms. The thermal degradation temperature and heat distortion temperature of the cured BMI-modified epoxy and CE-epoxy systems increased with increasing bismaleimide content. (C) 2003 Wiley Periodicals, Inc.