Irreversible hygrothermal aging effects on DGEBA/DDA epoxy resin were studied by FTIR, gravimetric tests, and DMTA. The results show that water immersion at 50, 70, and 90 degrees C leads to the introduction of carbonyl groups in the resin and chain scission of the crosslinked structure. These irreversible structural changes induce the addition of water to the polymer initially and weight loss of the material eventually. They also lead to the irreversible decrease of the glass transition temperature of DGEBA/DDA.