Blends of melt-mixed styrene - maleic anhydride and styrene - acrylonitrile copolymers were examined with respect to miscibility and free volume parameters. Differential scanning calorimetry, dynamic mechanical analysis, scanning electron microscopy, and positron annihilation lifetime spectroscopy were used to judge blend miscibility. It was found that blends containing copolymers with similar amounts of styrene were miscible. Positron annihilation lifetime spectra of all blends were evaluated with the POSITRONFIT and the MELT program, where the latter can be used to obtain lifetime distributions. The mean o-Ps lifetimes (or hole sizes) of the blends were found always to be between those of the pure constituents. Pronounced negative deviations of the o-Ps intensity from weighted averages were measured in miscible blends but not in immiscible ones. Immiscible blends exhibit, on the basis of MELT evaluations, unsymmetric and broader o-Ps lifetime distributions than do miscible blends.