An electrochemical cell has been developed forin situx-ray diffraction from a working electrode under clean conditions equivalent to ultrahigh vacuum conditions of5×10−10 mbar.The substrate crystals can be preparedex situand transferred into the cell under protection of ultrapure water within a few seconds. The oxygen level in the electrolyte is reduced by continuousN2flow to less than 0.2&percent; compared to that of a fresh electrolyte. This can be done while rotating the cell by 360° about the surface normal. The electrode potential is accurately measured at the position of the crystal using a Luggin capillary and a standard reference electrode. We demonstrate the performance of our cell byin situsynchrotron x-ray diffraction measurements on ultrathin Co layers electrodeposited on Cu(001) in an aqueousH2SO4/CoSO4solution. ©1998 American Institute of Physics.