Modulated differential scanning calorimetry has been carried out on melt-mixed blends of poly(ethylene oxide)/atactic-poly(methyl methacrylate) (PEO/PMMA). Two PEO molecular weights have been used to prepare blends in the concentration range 10 to 80 wt % of PEO. Two glass transitions temperatures were observed for the fully amorphous blends, in the 10 to 30 wt % PEO range, using the differential of heat capacity with respect to temperature [dC(p)/dT] signal. The semicrystalline blends, 40, 60, and 80 wt % PEO, exhibited melting of PEO crystallites and the PEO-rich phase glass transition at -30 to -50 degrees C. A second glass transition around 30 degrees C was detected for the 40 wt % PEO blend when a cooling run was carried out, because PEO crystallization was avoided under these conditions. Therefore, heterogeneous amorphous phases were observed not only for fully amorphous blends, but also for semicrystalline ones. Further analysis of the dC(p)/dT signal, obtained from the MTDSC experiments by fitting with Gaussian curves, showed that there is an interphase that varies in amount between 10 to 50 wt %. Correlation of the MTDSC observations with NMR spectroscopy and SAXS/SANS literature results are discussed.