The formation of thin oxide films on (111) and (110) single crystal specimens of FeCrAl based ODS alloys during the early stages of oxidation up to 1000 degrees C was investigated by atomic force microscopy (AFM) and secondary neutral mass spectrometry (SNMS). The AFM results demonstrated a crystalline character and terraced surface morphology of the alumina scale. The oxide scale morphology (height and oxide crystallite grain size) was only slightly dependent on the crystallographic texture of the underlying bulk material. The in-depth profiles determined by SNMS indicated that in this early stage of oxidation at 800 degrees C, the scale growth occurred by outward diffusion of aluminium from the scale/alloy interface to the scale/gas interface. The microstructure of the oxide scale and of the bull; material after long time exposure up to 1200 degrees C was quantitatively characterised by transmission- and scanning electron microscopy (TEM, SEM). The combination of AFM with conventional electron microscopy provides a powerful technique for microstructural investigation of oxide scales.