We have developed a new spectro-electrochemical cell derived from the three-electrodes standard cell, in order to study the electrolyte/single-crystal interfaces by using surface X-ray diffraction. Results obtained with this cell are described here. We studied the phase transition of gold (111) in sulfuric acid. Preliminary results are shown concerning the overpotential deposition and the new possibilities that are opened by the use of a 2-D X-ray-sensitive detector. This set-up allowed us to follow in situ and in real time the morphology of Cu growth on Au (001), in free and surfactant-containing solution.