The influence of blend composition on physical aging behavior was assessed for miscible blends of atactic polystyrene (a-PS) and poly(2,6-dimethyl- 1,4-phenylene oxide) (PPO). At aging temperatures of 15 and 30 degrees C below the midpoint glass transition temperature (T-g), the a-PS/PPO blends exhibited volume relaxation rates that were retarded compared to additivity based upon the aging rates for pure a-PS and PPO. This negative deviation diminished with increased undercooling, and eventually the volume relaxation rates displayed a nearly linear trend with respect to composition at the greatest undercooling of 60 degrees C that was employed. The compositional nature of unaged glassy density and secondary relaxation intensity, both influenced by the presence of specific attractive interactions in the blend system, were likely causes for the variation of volume relaxation rate with composition and undercooling. For aging at 30 degrees C below T-g, the dependence of enthalpy relaxation rate on composition was similar to that observed for volume relaxation. Mechanical aging rates determined from time-aging time superposition of creep compliance data showed significantly less than additive behavior for the blends aged at T-g - 30 degrees C, but unlike the volume relaxation results, this trend persisted at the 60 degrees C undercooling.