For isotope enrichment of chlorine, Cl2was excited by a 488‐nm line of the Ar ion laser and reacted with tetrachloroethylene. It was found that the reaction rate was limited to the excitation rate of Cl2by the laser. The enrichment factor of37Cl in residual Cl2increased linearly with the irradiation time and the result agreed with the approximate relation derived from the reaction rate. The enrichment factor of 1.17 was obtained after 120 min irradiation at the laser power of 180 mW.