In situ scanning tunneling microscopy (STM) and cyclic voltammetry have been employed to investigate the adsorption of eosin molecules on Au(1 1 1) in aqueous HClO4 solution. The eosin molecules are found to form a highly ordered adlayer on the well-defined Au(1 1 1) surface. A dimeric structure of eosin molecules is clearly resolved in highresolution STM image. Based on the STM observation and theoretical calculation, a structural model is tentatively proposed. The interaction between the bromide atoms is assumed to be responsible for the formation of the dimeric structure. (C) 2003 Elsevier Science B.V. All rights reserved.