The differential of heat capacity signal, dC(p)/dT, from modulated-temperature differential scanning calorimetry (M-T d.s.c.) was used to elucidate the miscibility and glass transition behaviour of poly(methyl methacrylate) (PMMA) and poly(epichlorohydrin) (PECH) blends. The conclusion reached was the same as that reported by Higgins and co-workers (Clark, J.N., Higgins, J.S., Kim, C.K. and Paul, D.R. Polymer 1992, 33, 3137) from small-angle neutron scattering, but different from that of Fernandes (Fernandes, A.C. Ph.D. Dissertation, University of Texas at Austin, 1986) from a d.s.c. study. This M-T d.s.c. study reveals that PECH/PMMA blends are not fully miscible. More detailed information was obtained from the dC(p)/dT signal. In this blend system, there were PMMA-rich, PECH-rich phases and interfaces, indicating that the blend is partially miscible. The breadth of the glass transition region, Delta T-g, can be used to judge polymer-polymer miscibility. For polymer blends, Delta T-g as a miscibility criterion was considered to obey the following relation : Delta T-g=w(1) Delta T-g,T-1+w(2) Delta T-g,T-2 where w(1) and w(2) are the weight fractions for polymer 1 and polymer 2, and Delta T-g,T-1 and Delta T-g,T-2 are the glass transition widths for polymer 1 and polymer 2, respectively. Partial miscibility results in a broad transition region. The width of the transition may be indicative of the presence of microphase separation and of interfaces. The M-Td.s.c. dC(p)/dT signal provides increased sensitivity and resolution for studying polymer-polymer miscibility and multiphase polymer materials.