Correlations between the oxidation processes and the mechanical properties have been examined for the Ni based alloy (Alloy 600). Heat treatments for 24 hours at 500 degreesC were carried out under "vacuum" and in air, both leading to oxidation of the metal. The effect of oxidation on the local mechanical properties has been studied on cold rolled specimens by means of microhardness tests performed with increasing load, in order to determine these properties in the depth of the sheet. The results show that air oxidation softens significantly the material, just under the oxide. The microstructures of the oxides formed in vacuum and in air were characterised by cross - section TEM and EDS analysis. The dislocation microstructures were observed on tensile strained specimens before and after heat treatment, using, respectively, electropolished and ion - milled thin foils. Slight differences in the dislocation distribution are reported; however, these results must be interpreted with great care due to possible effects of the thin foil preparation process, namely defect generation during ion milling.