The electrical performance of double layered Ohmic contacts not only depends on layer thicknesses, annealing temperatures and annealing time etc. but also Ohmic contact metals and reactions with substrates and each other. Furthermore, oxidation tendency of Ohmic contact metals is highly important for contact quality and stability. In this study, we have investigated Ti/Al Ohmic contact formation on undoped AlGaN substrates by X-ray photoelectron spectroscopy (XPS) depth profile analysis. We aimed to reveal the oxide compounds and reaction products of metals and dependencies to the contact depth. In survey mode and depth profile mode five atoms core level (Ga 3d, Al 2p, N 1 s, Ti 2p and O 1 s) was observed and the XPS core level peaks were convoluted with Gaussian profiles to identify the origin of the peaks. Our results show that Ohmic contact metals strongly incorporated with oxygen, leading to high specific contact resistivity (qs) confirmed by the TLM measurement. Also it is found that thermal treatments cause a TiAl3 metallic alloy in the Ti/Al interface and the presence of excess Ga and TiN indicates that out diffusion of N atoms at the metal semiconductor (MS) interface. (c) 2021 Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the International Congress on Semiconductor Materials and Devices, ICSMD2018.