Adlayers of toluene, p-xylene, o-xylene, and mesitylene on a Rh(111) electrode in HF solution were investigated by using in situ scanning tunneling microscopy (STM). In situ STM images showed that all molecules were adsorbed with flat-lying orientation on the Rh(111) surface. High-resolution imaging revealed not only the packing arrangement but also the internal structure of each molecule. Methyl functional groups on the benzene ring were clearly discerned. Toluene molecules were arranged nearly hexagonally on Rh(111). The benzene ring of a toluene molecule was assigned to a hollow site on Rh(111). p-Xylene molecules were adsorbed at bridge sites, and a highly ordered c(2root3 x 4)rect adlayer was formed. The o-xylene adlayer showed a nearly hexagonal arrangement of molecules, and benzene rings were located at bridge sites. Mesitylene molecules appeared triangular in the STM image, which were assigned to hollow sites. (C) 2002 Elsevier Science B.V. All rights reserved.