The structure of the electrochemical double layer at the interface between a Ag(111) electrode and 0.1M KOH electrolyte has been probed using in-situ surface X-ray scattering (SXS). Detailed modeling of the SXS data at negative potential (E=−1.0V versus SCE) is consistent with the presence of an hydrated K + cation layer at a distance of 4.1±0.3Å from the Ag surface and at positive potential (E=−0.2V), indicates that the presence of OH ad stabilizes the hydrated K + cations through a non-covalent interaction forming a compact double layer structure in which the Ag-K + distance is reduced to 3.6±0.2Å.