A proton-transferring aromatic bis-azo compound, 1,4-bis-p-sulfonylazo-2,3-dihydroxy naphthalene (SADN), and its possible binuclear copper(II) complexes have been studied in aqueous solution. Both ground- and excited-state prototropism of this compound is investigated by means of carbon-13 nuclear magnetic resonance, electronic absorption and fluorescence emission spectroscopies, and semi-empirical molecular orbital methods. The formation constants of both mono- and binuclear Cu I I complexes have been evaluated from potentiometric and spectrophotometric results. The possible structures of both free SADN and its complexes are discussed.