Positron annihilation lifetime (PAL) spectroscopy has been utilized to investigate the free-volume hole properties of two types of polymer blends, a miscible blend of tetramethyl-Bisphenol A polycarbonate (TMPC) and polystyrene (PS) and an immiscible blend of Bisphenol A polycarbonate (PC) and PS. Larger fractional hole volumes are observed in TMPC than in PC in order to form a miscible blend with PS. In miscible blends, the free volume shows a negative deviation due to blending. In immiscible blends, the relationship between the free volume as detected by positronium annihilation and the weight fraction is complicated due to the presence of interfaces. The free-volume hole distribution is additive in miscible blends, while a significant broadening is observed in immiscible blends. The observed negative deviation of the free-volume hole fraction in miscible blends is interpreted in terms of segmental conformation and packing between dissimilar polymers.