The influence of blend composition on volume relaxation behavior was determined for miscible blends of poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN). The volume relaxation rates displayed an approximately linear dependence on blend composition for all of the aging temperatures employed which were 15, 30, and 45 degreesC below T-g. No unique behavior was observed for the blends compared to the pure polymers in terms of glass transition fragility observed by differential scanning calorimetry or the variation of density with composition. The dynamic mechanical beta -relaxation process for PMMA was observed in the PMMA/SAN blends, and the intensity of the relaxation diminished with increasing SAN content. These results were contrasted with previous findings by the present authors on blends of atactic polystyrene (a-PS) with poly(2,6-dimethyl phenylene oxide) (PPO). Fragility, density, and secondary relaxation features of the PMMA/SAN and a-PS/PPO blends were influenced by the respective nature of interactions in these mixtures, and the volume relaxation rates were consistent with these characteristics. An apparent failure of time-aging time superposition was noted for PMMA due to overlapping alpha- and beta -relaxations.