In the past, austenitic stainless steels have become an important component in our everyday lives. Austenitic steels need a chromium content of 11 m% in order to be classified as ,,stainless’’. Chromium reacts with the surrounding atmosphere and creates an oxide layer of a few nanometers. This layer protects the steel from further oxidation and is responsible for the mechanical properties in corrosive media. The goal of this master thesis was to develop a method for analyzing oxide layers of austenitic stainless steels via atom probe tomography and to show differences in the passive layer of two types of austenitic stainless steels, a CrNi- and a CrMnN-alloy. A 3 µm silver layer was additionally applied to the surface of the steel to protect the oxide layer during the fabrication of the samples in the Focused Ion Beam (FIB) microscope and during alignment in the atom probe. Measurements were carried out in voltage and laser mode. The samples measured with laser had a more complex mass spectrum than those in voltage mode. A pulse frequency of 100 kHz is recommended in order to detect the heavy silver atoms properly. The measurements of the CrNi-alloy showed that the oxide layer is depleted in iron and nickel. Moreover, the amount of chromium, manganese and nitrogen is increased. Oxygen and carbon clusters were found below the passive layer. In contrast the oxide layer of the CrMnN-steel is depleted in manganese and iron and enriched in chromium, molybdenum and nitrogen. No clusters were found in this alloy.