The oxidation behaviour of four model CoReCr alloys and a commercial Co-based alloy was investigated at 1000 °C and a low-oxygen partial pressure of p(O2) = 10−16 bar, in order to prove the feasibility of a pre-oxidation treatment. Under suitable conditions of the pre-oxidation treatment, the oxidation of the highly reactive alloying element Cr is possible. All the studied alloys form a continuous and dense Cr2O3 scale on the metal surface. The transport of chromium to the surface occurs mainly from the Cr-rich σ-phase, which becomes completely dissolved in the surrounding matrix after long exposure times. As a result of the Cr2O3 scale, growth depletion of Cr occurs in the near surface region, leading to internal oxidation.