The kinetics of, and the factors responsible for, sensitization and de-sensitization were determined for duplex 308 stainless steel. Samples consisting of 10 vol.% of ferrite, balance austenite were heat treated at 600 and 550°C for times of 15 min-300h. The chromium contents of the α−γ boundaries and γ−γ grain boundaries of heat treated samples were measured by a potentiostatic electrochemical technique. Transmission electron microscopy of foils and surface replicas of heat treated samples was used to observe carbide precipitation on the boundaries and decomposition of the ferrite. Carbide precipitation occurred exclusively along the α−γ boundaries. Most of the chromium in the carbide came from the ferrite phase. However, intergranular corrosion susceptibility resulted from a narrow chromium depleted zone that was formed on the austenite side of the interface and a chromium depleted zone that formed along the γ−γ grain boundaries. The latter developed in spite of the absence of carbides along the γ−γ grain boundaries. It resulted from grain boundary diffusion of chromium to the carbides growing along the α−γ boundaries. There was a maximum in the degree of sensitization with time of heat treatment. At aging times < t max , the degree of sensitization was limited by the achievement of a chromium depleted zone of a critical minimum width. An upper bound estimate of the latter was ~ 25 Å. At aging times > t max , de-sensitization occurred. This was controlled by the chromium content in the matrix at the carbide-matrix interface. Initially, the rate of de-sensitization was rapid and was produced by chromium diffusing from the ferrite phase and into the chromium depleted zone in the austenite. When the ferrite phase started to decompose the rate of de-sensitization decreased dramatically and was produced by volume diffusion of chromium in the austenite phase.