We used X-ray diffraction to investigate the n-GaAs(001)/0.5M H2SO4 interface in-situ under potential control in a three-electrode, thin-layer electrochemical cell. The intensity of crystal truncation rods as a function of the electrode potential was recorded. A pronounced increase in surface roughness was proven by the strong decrease in the rod intensities with time at an electrode potential of -0.6 V versus SCE. Surprisingly, this process could be partially reversed at more negative potentials (< -0.9 V versus SCE). Possible mechanisms explaining these observations are discussed. After the deposition of Cu from a 0.5M H2SO4/1mM CuSO4 solution, three-dimensional, epitaxially grown Cu islands with several degree mosaic spread were observed.