Using a microthermal analyzer TA Instruments 2990 muTA, we have analyzed the morphologies developed for the resin tetraglycidyl-4,4'-diaminodiphenyl-methane cured with an aromatic amine 4,4'-diaminodiphenylsulphone modified with different amounts of poly(styrene-co-acrylonitrile) (SAN) thermoplastic, The phase-separation phenomenon induced by polymerization was also followed by scanning electron microscopy. Using the modulated local thermal-analysis mode of muTA, the glass-transition temperatures of different domains for each sample were evaluated. Dynamic mechanical analyzer experiments were made to evaluate the macroscopic thermal properties of the blends, A morphology was well established fur all blends examined with these techniques showing a nodular structure, the epoxy-rich phase, and a continuous phase, the SAN-rich phase, that forms the matrix. From both microscopic and macroscopic thermal analyses, it is concluded that a phase separation exists for the blends investigated.