Experiments have been completed in order to characterize the fatigue behavior of weld-bonded aluminum 5754-O/bis-phenol-A epoxy adhesive joints subjected to 100% relative humidity at 38-degrees-C. It was found that the presence of water vapor at elevated temperature decreases the fatigue strength of weld-bonded joints by as much as 33% at 5 x 10(6) cycles. Optical microscopy, scanning electron microscopy, dynamic mechanical analysis, and tensile testing of fatigued specimens and exposed bulk adhesive revealed that fatigue strength degradation is mainly due to the plasticization and micro-cracking of adhesive by the water vapor.