Positron lifetime spectroscopy was applied to study structure and interphase interaction in partially miscible polyearbonate/poly(styrene-co-acrylonitrile) blends. Orthopositronium (oPs) lifetimes and intensities for blends of different compositions were followed as functions of measurement time. Slight decreases in oPs intensities, caused by irradiation of specimens from positron source during measurement, were observed and oPs parameters, unperturbed by the irradiation, were deduced. These were plotted against the weight ratio of the blend components, In accord with expectations for partially miscible blends, they followed the straight lines of additivity of the free volume of the blend constituents.