The changes in physical and chemical states with time at the very late stage of the cure process in a thermoset/thermoplastic blend, triglycidyl p-aminophenol (epoxy)/poly(ether sulfone) (PES)/4,4’-diamino-diphenylsulfone (curing agent), were investigated by dynamic mechanical analysis (d.m.a.) and Fourier-transform infra-red analysis (FTi.r.). D.m.a. showed the two-phase character : two-step decrease in the dynamic storage modulus E’ versus temperature curve and two peaks in the dynamic loss tan delta versus temperature curve, indicating phase separation via reaction-induced spinodal decomposition (which was confirmed by scanning electron microscopic observation). The low-temperature tan delta peak assigned to the T-g of the PES-rich phase was elevated and the high-temperature peak assigned to the T-g of the epoxy-rich phase was depressed with increasing cure time. The inward shift of the two T-g values was quite strange. This strange shift was interpreted by taking account of the segregation of epoxy oligomer from crosslinked epoxy network in the neat epoxy system (probably by micro-gel formation) and the chain scission of the epoxy network established via etherification. That is, the low T-g of PES/epoxy oligomer mixture shifts to a higher temperature to attain the T-g of neat PES by converting the oligomer to the network, and the high T-g of the segregated pure epoxy network shifts to lower temperature by chain scission.