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Atomic layer deposition (ALD) is the most advanced technique for the fabrication of ultra-thin conformal films. To yield high quality films, the knowledge of chemical reactions and interactions between the substrate surface and the precursors is becoming increasingly important, especially within the very first ALD cycles. In this work, the ALD process of aluminum oxide with trimethylaluminum (TMA) and water is studied by using X-ray photoelectron spectroscopy (XPS) without vacuum break. This allows the investigation of the initial gaseous-solid-reactions, i. e. the chemisorption mechanism of the precursor molecules, with sub-monolayer resolution. The results show the ligand exchange during the ALD reactions and the dependence of the growth mode on the presence of hydroxyl groups and oxygen as adsorption sites on the substrate surface.