The adhesive fracture energy, G(c) , of aluminium alloy and steel joints bonded with a rubber-toughened epoxy adhesive has been measured using monotonically loaded tests. Such tests have been conducted at different levels of relative humidity, and two surface pretreatments have been employed for the substrates prior to bonding: a simple grit-blast and degrease (GBD) pretreatment or a silane primer (GBS) pretreatment. When G(c) was plotted against the crack velocity, three regions of fracture behaviour could be distinguished. At low rates of displacement the crack grew in a stable manner, visually along the interface, and relatively low crack velocities could be readily measured. This was termed "Region I," and here the value of the adhesive fracture energy was relatively low and decreased steadily as the relative humidity was increased. On the other hand, at relatively high rates of displacement the crack grew in a stick-slip manner mainly cohesively in the adhesive layer at approximately 20 km/min. This was termed "Region III," and here the value of G(c) was relatively high and independent of the relative humidity. In this region the crack was considered to grow faster than the water molecules were able to reach the crack tip, which explains the independence of G(c) upon the test environment. In between Region I and Region III, a transition region was observed, which was designated "Region II." The major effect of the GBS pretreatment, compared with GBD pretreatment, was to increase the value of G(c) both in Regions I and III, although the presence of the silane primer had the greater effect in Region I.