An AA5083 aluminium alloy sensitized in service after 40 years exposure to ground atmosphere temperature is studied. Nitric acid mass loss test (NAMLT) is used to determine the susceptibility to intergranular corrosion (IGC). The degree of sensitization in various areas through the alloy thickness was found different, which can be associated with non-homogeneous Mg distribution through the alloy plate thickness. Structure characterisation confirmed that the in-service sensitization is associated with the formation of the ' phase and a cubic Al-Mg transition phase with magnesium content between the GP zones and the '' phase at the grain boundaries. In order to simulate the in-service sensitization process and to gain insight into the sensitization mechanism, the sensitization of AA5083 alloy at relatively low temperatures, namely 70 and 100°C, is studied. For the AA5083 alloy sensitized at 70°C, although the mass loss value is below 15 mg/cm2, ' phase is observed as individual precipitates at grain boundaries. The AA5083 alloy after exposure to 100°C for 240 hours is susceptible to IGC since the ' precipitates have grown continuously at the grain boundaries. Additionally, the effect of sensitization in AA5083 alloy on stress corrosion cracking (SCC) is also investigated using constant displacement double cantilever beam (DCB) testing. It is found that the cracking length increases with the degree of sensitization. The population density of the crack branches also increases with the degree of sensitization. The metal between different small branches is known as ligament. And with high degree of sensitization, the ligaments between crack branches have become brittle. Therefore, small branches became connected to form a continuous crack with the crack propagating.