Previous work on the characterization of interphase regions in thermosetting composite systems has focused on the inference of an interphase layer from effects noticed through macroscale mechanical and thermal testing. With the development of atomic force microscopy and active thermal probes for this technique, it is now possible to examine material thermal properties on a much smaller scale. Variations in microscale thermal properties of an aerospace-grade thermosetting resin system were evaluated for car;bon and glass fiber reinforcement, using the modulated local thermal analysis mode of a TA Instruments 2990 mu TA. The variations observed clearly demonstrate the presence of a soft interphase layer in the glass material and underline the importance of fiber-matrix interactions during the formation of the interphase.