Blend samples were prepared by physical blending of nonglycidyl-type epoxy resin with varying weight ratios of carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer ranging between 0 and 25 wt%. The blend samples were cured with stoichiometric quantity of diamine. A comparative study of prepared blend samples by Fourier transform infrared (FTIR) spectroscopy showed the modification as a result of chemical reactions between cycloaliphatic epoxy, CTBN, and 4,4'-diamino diphenyl sulfone. The impact strength of the blend systems increased up to 15 wt% addition of CTBN in the blend and decreased thereafter. The toughness and elongation at break increased, whereas tensile strength decreased up to 20 wt% loading of CTBN in the blends. Two-phase morphology appeared with the incorporation of CTBN in the cycloaliphatic epoxy resin matrix. (C) 2008 Wiley Periodicals, Inc.