Micro-sized spherical Al particles have recently attracted interest for the development of a new concept for coatings based on their capability to form hollow alumina spheres and aluminized diffusion zones in the substrate. For understanding better their oxidation behaviour, spherical µm-Al particles with different sizes were oxidized in air on heating up to 1300°C and under isothermal conditions at 800°C and 850°C. The oxide formation was studied in situ by high temperature X-ray diffraction and the oxidised particles were analysed by scanning electron microscopy. On heating the µm-Al particles begin to form a g-Al2O3 scale before reaching the melting point and the molten Al is kept within the g-Al2O3 shell. On further heating q-Al2O3 is detected, which forms simultaneously with the g-Al2O3. The g-Al2O3 / q-Al2O3 scale is stable and protective under isothermal conditions up to 800°C within the investigated times. On further heating the g-Al2O3 and q-Al2O3 transform simultaneously to a-Al2O3 in a temperature range of 850°C to 1100°C. Under isothermal conditions the g à a-Al2O3 transformation is observed after 160 min at 850°C. During the g à a-Al2O3 transformation shrinkage occurs that leads to formation of pores. A model is proposed describing the mechanism that leads to the formation of the observed whiskers morphologies during the g à a-Al2O3 transformation.