An in-situ surface X-ray differential diffraction (SDD) in transmission geometry has been applied for the first time to the study of silver adlayer structure on Au (111) in the underpotential region. Hitherto surface differential diffraction has been used to determine vertical positions of adsorbates from the substrate, but has given no information upon the lateral position of the adsorbate relative to the substrate. By conducting experiments using a ((1) over bar 11) reflecting plane, we show that it is possible to determine the adsorption site in three dimensions via SDD. The studies confirm that silver is adsorbed on multiple sites in a sulphate electrolyte whereas a single adsorption site was found for both acetate and fluoride electrolytes.