In this thesis a new approach to investigate oxide-liquid interfaces at an atomic level by the combination of surface science characterization tools and in-situ experiments under environmentally relevant conditions is presented. Well defined single-crystalline iron oxide thin films - FeO(111) and Fe3O4(111) - were prepared and analyzed in ultra-high vacuum (UHV) by low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) and subsequently transferred to aqueous solutions. To investigate the electrochemical behavior cyclic voltammetry (CV) experiments were performed, which were supplemented by in-situ scanning tunneling microscopy (STM) and sum frequency generation spectroscopy (SFG). Afterward, the sample was transferred back to the UHV environment and an analysis of the thin film composition was conducted. This work is divided into four parts - theoretical fundamentals, methodology and two chapters containing experimentalresults. The first results chapter deals with the preparation and characterization of FeO(111) and Fe3O4(111) on Pt(111) in UHV and their stability in air, water and aqueous NaClO4 solutions at various pH values. It was found that both substrates exhibit a remarkably high stability,proving the chemical inertness under such reactive environments. In pH-neutral electrolytes no redox reaction could be observed for the two thin films, whereas at strongly acidic conditions (pH 1) the dissolution of both substrates was detected, for which a model was proposed.In the second results chapter the adsorption of three differently functionalized organic molecules - methanol, catechol (1,2-dihydroxybenzene) and 4-aminophthalic acid (4-APA) - on FeO(111) and Fe3O4(111) from liquid phase was tested and their influence on the iron oxide stability was examined. The behavior of the latter two substances was compared to corresponding UHV adsorption studies. Depending on substrate and chemical nature of the molecule, different adsorption behavior and thermal stability of the adsorbates were observed. The highest affnity of binding to functional organic compounds was found for Fe3O4(111), which is because of the eased accessibility of iron centers for complexation. For both iron oxide thin films no electrocatalytic activity toward methanol oxidation nor any electron transfer reaction to 4-APA were detected. In catechol-containing solutions electrochemical experimentsrevealed an intense quasi-irreversible redox reaction both on the iron oxide and on Pt(111). Both thin films were found to be stable in the presence of all three organic molecules, though, the dissolution of FeO(111) takes place during the CV sweep in a pH-neutral electrolyte in thepresence of catechol and 4-APA, respectively.