In the present work the structure of two different tool steels is examined before and after oxidation up to 1000 degrees C in air. The materials under examination have different chromium contents. Also, the first contains vanadium (S1 tool steel) and the second tungsten (S2 tool steel) as alloying element, while the rest are common. The examination took place by thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. From this study it is deduced that the structure of the two steels, before oxidation, has several distinguishing differences mostly in the chromium distribution in the iron matrix. The oxidation tests revealed that S2 oxidizes at higher temperatures than S1, but finally, at 1000 degrees C, S2 tool steel has greater mass gain, because it oxidizes at a higher rate. (C) 2009 Elsevier B.V. All rights reserved.