A self-assembled monolayer of l-cysteine was prepared on Au(111) under potential control. Cyclic voltammetry and in situ electrochemical scanning tunneling microscopy were employed to investigate the molecular adsorption and adlayer structure in perchloric acid solution. The molecules chemically adsorb on Au(111) and form a well-defined adlayer. A new structure of (4 × √7)R19° was observed in the double-layer region. A phase transition at the positive potential region resulted in the appearance of a disordered layer. A structural model is proposed to interpret the molecular registry with Au(111) substrate.