HgCl2 can disperse onto the surface of activated carbon as a monolayer to form a catalyst for making vinyl chloride from C2H2 and HCl. A monolayer dispersion capacity of 0.020 g. HgCl2/100 m2 activated carbon was determined by x‐ray diffraction. In order to understand further the catalytic mechanism of this kind of catalyst, the dispersed structure of HgCl2 supported on highly oriented pyrolitic graphite (HOPG) has been studied by scanning tunneling microscopy (STM). The HgCl2–HOPG sample is obtained by putting a drop of dilute HgCl2 solution on the surface of HOPG and drying in air. The HgCl2–HOPG surface is imaged by computer controlled STM under ambient conditions and at room temperature. In constant‐current mode, bright spots with regular structure over the periodic structure of HOPG has been observed in the image. The size of bright spot is about (4×9) Å which corresponds to a single HgCl2 molecule. It suggests that HgCl2 molecules form a monolayer on the surface of HOPG with a (3.6×3.6) R14.3° surface superlattice in contrast to the lattice of HOPG. The experimental result is consistent with the estimation derived from the monolayer dispersion theory.