Adsorption of benzene on well-ordered Rh(111) and Pt(111) electrodes was investigated in HF solution using in situ scanning tunneling microscopy (STM). Ordered adlayers of adsorbed benzene with a c(2 root 3 x 3)rect-2C(6)H(6) (theta = 0.17) symmetry were found to form on both electrodes in the double-layer charging region. A phase transition was observed in the benzene adlayer at negative potentials, where partial desorption of adsorbed benzene took place. The c(2 root 3 x S)rect structure was transformed into (3 x 3)-1C(6)H(6) (theta = 0.11) and (root 21 x root 21)R10.9 degrees (theta = 0.14) on Rh(111) and Pt(111), respectively. High-resolution STM images obtained on Rh(111) revealed the internal structures of benzene molecules adsorbed at different binding sites in both phases. They appeared as triangular and dumbbell shapes in the (3 x 3) and c(2 root 3 x 3)rect structures, respectively, implying their 5-fold and 2-fold registries. The 2-fold bridging coordination was thought to occur in the (root 21 x root 21)R10.9 degrees phase on Pt(111).