In situ scanning tunneling microscopy (STM) was used to probe the adlayer structures of benzene, naphthalene, and anthracene on a well-defined Cu(lll) electrode surface in aqueous HClO4 solution. These molecules were found to form highly ordered adlayers on a clean Cu(lll) surface with a flat-lying orientation. The molecular orientation and packing arrangement were also determined. The ordered benzene adlayer was observed to form a (3 x 3) structure with a characteristic triangular shape for each benzene molecule. The naphthalene adlayer was observed with a (4 x 4) symmetry. The naphthalene molecules were preferentially aligned with their long Ct axes along the atom rows of the Cu(lll) substrate. A long range ordered structure was also found for the anthracene adlayer. Straight molecular chains formed along the atomic rows of Cu(lll) with a side-by-side configuration.