Atomic layer deposition (ALD) has been established as the main technique for creating MOS structures based on III-V semiconductors, but still device performance is limited by the quality of the interface towards the high-k oxide layer. X-ray photoemission spectroscopy (XPS) is a well-suited tool for analyzing the structure and chemical composition of such interfaces. However, conventional XPS studies under ultrahigh vacuum conditions can only compare the situation before and after individual half-cycles of the ALD process. Here I will show how ambient-pressure XPS can be used to study surface structure and chemistry live and in-situ during the ALD reaction. As an example, we have investigated the deposition of HfO2 on InAs using TDMA-Hf and water as precursors, revealing several steps within the chemical reaction of InAs exposed to the Hf precursor material, which we interpret as a temperature-dependent adsorption of unreacted precursor molecules preceding the ligand exchange reaction.