Blends of 4,4’-diaminodiphenylmethane-crosslinked diglycidyl ether of bisphenol A/poly(ethylene oxide) (PEG) were investigated. The miscibility of the blends was established on the basis of thermal analysis and dynamic mechanical analysis studies. A single, composition-dependent glass transition temperature (T-g) was seen, and a marked negative deviation from the Fox equation was noticed after the curing reaction. The more PEO content in the blends, the larger is the negative deviation. This is ascribed to the formation of imperfect semi-interpenetrating polymer networks of epoxy/PEO, since the dilution effect of PEG, which is a miscible, inactive diluent, results in an incomplete curing reaction. Therefore, T-g values decrease.