Duplex stainless steel exhibits a dual phase microstructure of austenite islands in a ferrite matrix. The oxidation characteristics of the phases depend on the composition within them. High temperature, in situ SEM observation of the growing oxide on a duplex stainless steel has shown differences in the morphologies of the oxide on the two phases. Cross-sections through the oxide, and at phase boundaries reveal that the structures and thicknesses of the oxides on the two phases differ and that there is enhanced oxide growth at the phase boundary which may be due to enhanced diffusion of manganese.