The interaction between MgCl2 and SiO2 was investigated by X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS) and contact potential difference (CPD) measurements. A thin SiO2 layer was grown for this purpose on a Si(100) wafer and MgCl2 was applied on this support at room temperature by evaporation under UHV conditions. It was found that magnesium chloride is deposited molecularly on the SiO2 substrate, growing in layers and covering uniformly the oxide surface. The interaction with the substrate is initially very weak and limited to the interfacial layer. Above 623 K, most of the molecular MgCl2 is re-evaporated and the interfacial interaction becomes stronger, as Mg-Cl bonds in the remaining sub-monolayer chloride break and Cl atoms desorb. This leaves on the surface sub-stoichiometric MgClx, which interacts with oxygen atoms from the substrate to form a complex surface species. At 973 K all Cl atoms desorb and Mg remains on the surface in the form of an oxide. (C) 2009 Elsevier B. V. All rights reserved.