The underpotential deposition of Ag on Au(111) was used to demonstrate the adsorption of monolayers at the electrodeelectrolyte interface and their measurement with X-ray diffraction in situ and in real time. The information obtained is that of the adlayer spacing during the monolayer formation with respect to the interplanar spacing in the bulk of the substrate. Although it was not straightforward to assign the spacings of the Ag adlayer to adsorption sites on the Au(111) electrode, it was possible to observe phase transitions in the adlayer during underpotential deposition. Detailed theory of the differential X-ray diffraction phenomenon and data analysis is given. A conventional laboratory source of X-rays was used.