In situ scanning tunneling microscopy (STM) revealed that highly ordered adlayers of fullerene monomer (C-60) and [2 + 2]-type dimer (C-120) were formed on Au(111) by immersing the surface in benzene solution containing C60 or C 120 molecules. High-resolution STM images allowed us to determine packing arrangements of C-60 and C-120 on a Au(111) surface in an aqueous HClO4 solution. The (2root3 x 2root3)R30degrees and the so-called in-phase structures of the C-60 adlayer were found on the Au(111) surface, whereas the (2root3 x 4root3)R30degrees and (2root3 x 5root3)R30degrees structures were observed for the C-120 dimer. Each C-120 dimer was imaged in a dumbbell shape by in situ STM.