A molecular-scale understanding of the adsorption of metal ions on oxide surfaces is of fundamental importance for various scientific areas where the oxide/water interface plays a central role, such as corrosion science, preparation of heterogeneous catalysts, transport of contaminants in the environment. As a matter of fact, a fine-tuning of metal speciation during the first steps of heterogeneous catalyst preparation (oxide/water interface) will strongly control the final characteristics of the catalyst (activity/selectivity/stability).[1] However, a rational description of sorption mechanisms is generally made difficult by the ill-defined surface structure of high surface area oxides exposing a number of different sorption sites.[2] One way to mitigate this problem is to simplify the sorption system by using oriented single crystals that have a limited number of well-defined surface sites. This reductionist approach applied to aqueous deposition is almost inexistent in the field of surface science where most of the model studies use nonaqueous deposition techniques such as metal evaporation.[3] However, pioneering works of the research groups of Brown, Jr.[4] in the field of earth sciences and Niemantsverdriet,[5] Regalbuto,[6] or more recently Freund[7] in the field of catalysis have shown the tremendous importance of developing a surface science approach using a realistic aqueous phase deposition method. Aqueous deposition of NiII complexes on a-Al2O3 single crystals has been chosen in this work since this system has practical implications in heterogeneous catalysis for hydrotreating (removal of S, N, O, and metals from crude oil in refineries), hydrogenation, or steam reforming purposes.[8] a-Al2O3 single crystals are commercially available in two orientations, (0001) and (11¯ 02), exposing different types of surface sites partly mimicking the complexity of the surface chemistry of g-Al2O3, the standard catalytic support, that is not available as single crystals.[9] Moreover, both (0001) and (11¯02) a-Al2O3 orientations have been extensively characterized in the presence of water.[10] However, the use of single crystals with low surface area requires characterization of very low quantities of adsorbates. Hence, grazing-incidence X-ray absorption spectroscopy (GI-XAS) was chosen as the main molecular-scale characterization technique, since detection of a small amount of adsorbates is made possible by enhancement of the fluorescence intensity in the grazingincidence geometry.[11] Furthermore, the combined use of GI-XAS and oriented single crystals provide additional structural information on the local environment of the absorbing atom thanks to the synchrotron beam polarization. Actually, the measured number of neighbors (Nmeasured) will be three times higher than the actual number of neighbors (Nreal) when the chemical bond (directed along~r) is parallel to the electric field vector~e[Eq. (1), cosq=1].[12]