In situ scanning tunneling microscopy (STM) was employed to study adlayer structures of naphthalene, naphthoquinones, and anthracene on well-defined Rh(111) and Pt(111) electrodes in aqueous HF solutions. All molecules were adsorbed with the flat-lying orientation on both electrodes. Highly ordered adlayers of naphthalene and naphthoquinones were found to form on Rh(111) with the (3 root 3 x 3 root 3)R30 degrees structure, while disordered structures were found on Pt(111). Anthracene formed almost close-packed adlayers without long-range ordering. In situ STM disclosed not only the adlayer structure and packing arrangement but also the internal structure of each aromatic molecule. Two- and three-ring structures were clearly discerned for naphthalene and anthracene, respectively. An additional spot was observed for 1,2-naphthoquinone, which was attributed to the oxygen atom at the 2-position.