The underpotential deposition of Cu on Au(111) was studied with X-ray diffraction in situ and in real time. Potentiodynamic and potential step modes of deposition were used and the adlayer spacing of Cu on Au(111) was determined as a function of potential and time respectively. In the latter case, a structural transition of the Cu adlayer was observed, consistent with changing from adsorption predominantly in the atop position to three-fold hollow sites on Au(111). A conventional laboratory source of X-rays was used.