A free volume probing technique, positron annihilation lifetime spectroscopy (PALS) is used to characterize miscible blends of amorphous polycarbonate with a semicrystalline copolyester at room temperature. The paper describes the analysis required to use the PALS technique to probe the amorphous regions in which the miscibility of the blends occurs. Analysis of the raw PALS data in this system involves taking account of the influence of crystallinity on the PALS results. The results demonstrate that the PALS technique is able to discriminate between free volume in the amorphous regions of as-molded and annealed samples. This difference in free volume between samples of differing thermal history is assigned to the influence of polymer crystals on the stress state of the miscible amorphous regions.