The physical aging process of 4,49-diaminodiphenyl sulfone (DDS) cured diglycidyl ether bisphenol-A (DGEBA) blended with a medium molecular weight (i. e., M n = 10 600 and M w /M n = 2.3) poly(ether sulfone) (PES) was studied by DSC at four aging temperatures. The cured epoxy-rich phase is immiscible with the PES-rich phase, with their respective T g values being 29 8C apart. Hence, aging the network blend at a suitable temperature caused two well separated enthalpic relaxation processes due to physical aging of PES-rich and epoxy-rich phases. The experimental results indicated that the aging phenomena of the well separated phases in the network blend are similar to that of the corresponding neat polymers when the aging temperature T a is lower than T g by the same value.