Changes in mechanical properties during isothermal physical aging were investigated for three miscible blends: polystyrene ( PS ) / poly ( 2,6-dimethyl 1,4-phenylene oxide ) ( PPO ) , PS / poly ( vinylmethylether ) ( PVME ) , and poly ( methylmethacrylate ) ( PMMA ) / poly ( ethyleneoxide ) ( PEO ) . The kinetics of stress relaxation was investigated for the blend, dilute in one component, and compared with that of the neat major component at equal temperature distances, T g -T, from the midpoint glass transition temperature. It is demonstrated that for all three blends, the mean stress relaxation time ยป tโฆ does not scale with T g -T . For PS / PPO and PS / PVME blends, the stress relaxation rates are faster compared to neat PS; for PMMA / PEO, they are slower than for neat PMMA. Two effects appear to be important in contributing to this discrepancy. First, addition of the second component produces a change in the packing density of the blend: less dense for PS / PPO and PS / PVME; more dense for PMMA / PEO. Comparison of average free volume hole sizes and fractional free volumes measured via orthopositronium annihilation lifetime measurements for all three blends versus the pure constituents are qualitatively consistent with this interpretation. Second, because of the presence of concentration fluctuations in the blend, it is expected that the initial stress decay is dominated by regions enriched in the more mobile component. From observations of the change in width of the stress relaxation time distribution, this effect appears to be particularly significant in the PS / PVME blend.