Novolac epoxy resins cured with novolac resin, novolac acetate resin, novolac butyrate resin, and novolac phenylacetate resin named as EP, EPA, EPB, and EPP, respectively, were prepared. Their physical aging behavior at a T-g-30 degreesC (30 degreesC below glass-transition temperature) was examined by positron annihilation lifetime spectroscopy and differential scanning calorimetry. The ortho-positronium annihilation lifetime tau(3) variation extent of EP is less apparent than that of the other three esterified samples during physical aging. The time dependence of ops intensity I-3 agreed with the Kohlrausch-Williams-Watts (KWW) equation. The relaxation time (tau(o)) and nonexponential parameter were calculated. The free volume and enthalpy relaxation rate characterized by the reciprocal of tau(o) and partial derivativeDeltaH/partial derivativelogt, respectively, exhibit the same order-EPP > EPB > EPA > EP. These results suggest that the extend and rate of relaxation are not only related to the frozen free volume produced by quenching but also significantly influenced by segmental mobility of the network that attributed to the side-group flexibility and their interaction with networks. This work also supports the fact that side-group flexibility and the free-volume fraction and distribution act in concert to control the water-diffusion behavior in epoxy networks. (C) 2003 Wiley Periodicals, Inc.