Epoxy resins (EP) poly(ethylene oxide) (PEO) blends cured with phthalic anhydride were studied by differential scanning calorimetry and dynamic mechanical analysis. Single glass transition temperatures were observed for all blends before and after curing, indicating a high degree of miscibility. At the same time, a marked deviation from the empirical equation (e.g. the Fox equation) was noticed after curing. This is ascribed to the dilution effect of the PEO component and participation of PEO in the cure reaction resulting in incomplete crosslinking, i.e. the formation of imperfect crosslinking network structures. Fourier transform infra-red spectroscopy provided strong evidence that there is a specific interaction between the EP and PEO molecules.