Adsorption of 4-(4-nitrophenylazo)-1-naphthol (NPAN), an azobenzene derivative, on Au(111) has been investigated in aqueous HClO4 solution by using the cyclic voltammetry and scanning tunneling microscopy (STM). The molecule is found to form a stable monolayer on the electrode with a (6×4) structure relative to the underlying Au(111) lattice. Cyclic voltammograms show that the redox reactions occurring on the electrode are retarded by the formation of the monolayer. A structural model is proposed for the two-dimensional adlayer.