Phase modifications in a partially miscible blend PS/ PMMA and an immiscible blend PVC/EVA have been brought about using electron beam and microwave irradiation to improve the interfacial tension. These changes have been characterized by positron lifetime technique through free volume monitoring with support from DSC measurements. The hydrodynamic interaction parameter deduced from free volume data turns out to be a good parameter to characterize the interfacial changes compared with free volume data itself. The free volume data also confirms that the conventional DSC measurements are not adequate to differentiate the level of miscibility or interfacial tension. Present data based on hydrodynamic interaction parameter indicate that microwave irradiation is a better route to stabilize the interfaces than e-beam irradiation.