Two approaches were adopted to examine the irreversible interactions between water and a model epoxy resin (DGEBA/DDA). One was to employ XPS to investigate the changes of the chemical structure of the resin after immersion in water at a temperature of 90 degrees C and followed by completely drying. The other was to use XPS and FTIR to characterize the degradation products formed during hygrothermal aging. The results clearly showed that irreversible interactions occurred between water and the resin at elevated temperature. Water can lead to the incorporation of C-O and N-CO-N groups in the resin and cut the epoxy backbone chain. Based on the results obtained, a tentative degradation mechanism is proposed and found to explain the results reasonably well.