The surface symmetry of, and the polarized-laser-induced reactions on, Si (111)/Cl2 surfaces have been studied using second-harmonic generation (SHG). The outermost surface originated SHG signals, generated by 1.1, 1.3 and 2.0 eV probe, show the modified 3 m symmetries enhanced along the [21¯1¯] direction, probably because of a macroscopic asymmetric surface structure along the [21¯1¯] orientation. On the other hand, the SHG signals originated by the direct bulk transition, generated by 1.6 and 1.7 eV probe, show good 3 m symmetries rather reflecting the symmetry of bulk Si. The threshold power densities of the laser-induced Si (111)/Cl reaction showed significant dependence on the pump polarization with respect to the crystal orientation of the sample surface, and no important dependence on that with respect to the plane of incidence. © 1998 American Vacuum Society.