Structure of the adlayer of p-xylene on Rh(111) in HF solution was investigated by the complementary use of in situ scanning tunneling microscopy (STM) and ex situ low-energy electron diffraction (LEED). In situ STM images revealed that p-xylene molecules lie flat on the Rh(lll) surface, forming a highly ordered adlayer with a c(2 root3 x 4)rect structure. High-resolution imaging revealed not only the packing arrangement but also the internal structure of each molecule. Two methyl functional groups at the para position of benzene were clearly discerned. The adlayer structure obtained by LEED was consistent with that determined by in situ STM.