Plasma oxidation represents a good alternative method obtaining low temperature thin films. Thin Al-coatings were plasma treated in a 2.45 GHz microwave oxygen discharge to study the oxidation kinetics. The films were investigated by grazing incidence x-ray reflectometry (GIXR) and grazing incidence x-ray diffractometry (GIXRD). The oxidized surface was characterized by x-ray photoelectron spectroscopy (XPS) and IR-spectroscopy. The plasma formed alumina is x-ray amorphous. To quantify the alumina formation the change of the Al (111) integral intensity of the non-reacted part of aluminium was determined time-controlled. These values in connection with the total thickness of the film make it possible to give information about the growth behaviour of Al2O3. To determine the activation energy for plasma oxidation this process was investigated at different temperatures. The growth kinetics depend on the plasma type and power. Two processes oxidation and etching influence the film growth.