Transmission electron microscopy (TEM) has been used to study intergranular corrosion of aged AI-4 Wt% Cu by direct observation of identical regions of grain boundary before and after immersion in aqueous corrosion environments containing KCI + K2CrO4 and KCI + K MnO4. The experiments were conducted on differently aged material and correlated with experimentally determined values of intergranular susceptibility of aged rod material. In addition, a statistical determination was made of changes in grain boundary precipitate length, coverage, and density as a function of ageing time. All experimental results and observations were shown to be consistent with a simple theory of intergranular corrosion based on the location and extent of copper depleted anodic dissolution sites. Furthermore, similarities were shown to exist between intergranular corrosion and intergranular stress corrosion cracking (SCC) of AI-4 Wt% Cu.