The adlayer structure of 1,10-phenanthroline (phen) on Cu(111) in HClO4 was investigated using in situ scanning tunneling microscopy (STM) as a function of the electrode potential. Phen molecules were adsorbed almost vertically with their nitrogen atoms facing the Cu(111) substrate and stacked to form polymer-like chains. High-resolution images revealed each phen molecule as three bright spots corresponding to one benzene and two pyridine rings. At positive potentials, the molecules were packed closely to form ordered monolayers. At negative potentials, domains of adsorbed phen molecules changed continuously with time on the Cu(111) surface. (C) 2003 The Electrochemical Society.