High-temperature oxidation behaviour was studied for two ferritic stainless steels manufactured by powder metallurgy (P/M): AISI 409 Nb and AISI 434L. These steels find application in the manufacture of components exposed to high temperature in the automobile industry (exhaust pipes) and turbocompressors. Both steels were compacted at 700 Mpa and sintered in vacuum at three different temperatures (1225, 1250 and 1275degreesC). In this way, materials with different densities were obtained and, therefore, with different properties associated to this magnitude. Dimensional change was evaluated. A complete microstructural study was carried out, evaluating also porosity. Oxidation rate was measured by mass gain in an isothermal experiment in air atmosphere at 800degreesC for 48 hours. The oxidized samples were studied by X-ray diffraction and scanning electron microscopy to determine the influence of sintering temperature in the nature and structure of the scales. Obtained results make possible to see the important influence that the composition of the metal base and the porosity have in the oxidation behaviour at high temperatures of stainless steels manufactured by P/M.