One cause of long-term dimensional changes in glassy polymers is the gradual evolution of the viscoelastic behavior through aging processes in the glassy state. Many applications involve changes in relative humidity (RH), under which the materials exhibit aging processes that may differ from those in constant RH conditions. In this work, we report results from a study of a glassy epoxy subjected to isothermal RH-jumps. Similar to the temperature jump experiments of Kovacs, 6 we obtain the volume recovery responses in different histories as intrinsic isopiestics (constant RH), memory effect, and the asymmetry of approach. Our results qualitatively support the hypothesis that water has an effect similar to that of temperature on the structure (volume, enthalpy) of the glass-forming material. However, quantitatively, at the same RH - temperature states, the glasses formed by RH-jumps are different from those formed by temperature jumps. The implications of this difference for structural recovery are discussed. We have also extended the TNM-KAHR model to fit the experimental results of isothermal RH-jump experiments, and the outcome is also discussed in this paper.