Powder metallurgical (PM) stainless steels can be used for high-temperature applications. However, their characteristic porosity dramatically affects the resistance of the stainless steels to the oxidative attack and modifies the oxidation mechanisms. In this work, it is discussed how the processing parameters of PM stainless steels can modify the diffusion process when the material is exposed at high-temperature in oxidative environments. Processing parameters affect not only the amount but also the nature of the formed oxides. For powders of a given composition, the pressing method, the sintering atmosphere (vacuum, 100% H2 or 75%H2/25%N2) and the sintering temperature can modify the amount of porosity and its shape, often promoting the formation of less-protective oxides, instead of chromia. The different oxygen partial pressures in the inner pores and on the outer surface of the material also tend to make oxides formed inside the stainless steel pores more protective than those formed on their surface. X-ray diffraction, SEM and EDS studies of surface and cross-sectional views of the oxidized materials are used to prove these differences.