Comparison is drawn between data obtained from dielectric relaxation and thermally stimulated discharge current (TSDC) measurements on blends of carboxyl-terminated butadiene acrylonitrile (CTBN) with an epoxy resin (ER) obtained by reaction of the diglycidyl ether of bisphenol A (DGEBA) with either triethylenetetramine (TETA) or diamino-diphenylmethane (DDM). The effect of varying the composition of CTBN on the nature of the phase separation was examined using electron microscopy and was also evident from changes in the dielectric spectrum and the calculated activation energy for the molecular relaxation process. The morphology consists of spherical rubber particles dispersed in an epoxy matrix at low levels of CTBN, and at higher concentrations epoxy resin is observed to be precipitated as spheres within the rubbery phase. Detailed analysis of the dipolar spectra indicates that at low concentrations there appears to be some cosolution of the epoxy in the CTBN.