Chemical incorporation of a flexible molecule into a heat-cured epoxy offers an important route by which to toughen the epoxy and tailor its ultimate material properties. Acrylate-terminated urethane (ATU) flexibilizers containing polypropylene glycol have been employed in the present work to modify epoxies composed of a diglycidyl ether of bisphenol-A and bis(4-aminocyclohexyl) methane. Differential scanning calorimetry, dynamic mechanical analysis and electron microscopy reveal that the flexibilizer and epoxy undergo phase separation during cure when the flexibilizer loading or molecular weight is sufficiently high. Partial phase miscibility, deduced from both glass transition temperature shifts and stress-relaxation behaviour, is interpreted here in light of thermodynamic and free-volume considerations.