The initial stage of the reaction of aluminum trichloride (AlCl3) with the Si(111)-(7×7) surface and the annealing effects of the adsorbed surface are investigated with a scanning tunneling microscope (STM). Reacted and unreacted sites manifest in the contrast of adatom sites on the AlCl3-exposed surface. An AlCl3 molecule dissociatively adsorbs onto the Si( 111)-(7×7) surface at room temperature, which yields C1 atoms. Preferential adsorption site is found to be the center adatom site rather than the corner adatom site, and Cl atom adsorbs onto the top site of the center Si adatom. The migration of adsorbed molecules (AlClx) in the (7×7) unit cell is observed at room temperature. The SiCl,, species desorb from the surface by thermal annealing at 600 'C, leaving vacancies behind. After annealing at 1200 0C, Al deposition occurs in a limited area and the surface shows the V/7×V7 reconstruction, which is a characteristic Al-induced surface structure.