Epoxy adhesive bond durabilities were estimated by a constant displacement-rate method, based on both elastic energy release rate, G(1), and crack velocity. Tests conducted in humid air at 50 degrees C on adhesively-bonded aluminium alloy showed that G(1) depended strongly on load-point displacement-rate. The fracture energy of undegraded bonds, G(1C), the fracture energy for degraded bonds, G(1SCC), and a bond durability indicator, v(g), were each determined. G(1C) was higher for grit-blasted adherends anodized in phosphoric acid than for grit-blasted adherends treated with a silane coupling agent solution (SCA). Grit-blasted adherends treated with SCA formed more durable bonds than those anodised in phosphoric acid. X-ray photoelectron spectroscopy of fracture surfaces on SCA-treated adherends indicated that humidity had degraded the oxide film. Fracture surface inspection indicated that total adhesive failure did not necessarily imply low fracture toughness. This was explained in terms of a ＇＇plastic expansion＇＇ model.