Abstract: The thermal and radiolytic stability of CL-6-131I was compared with that of CL-19-131I and NCL-6-131I.Results showed that there was no significant dehalogenation even when CL-6-131I were being stored at 30℃, 45℃, 70℃ for 48, 48, 26 hours respectively, and it is still stable in pH=1, pH = 12 at 15℃, 60℃ 80℃ for a period of 3 to 15 hours. A solution of CL-6131I (15-20 mCi/ml) stored at room temperature for two months, gave the same TLC result as that of the newly prepared product.A solution of CL-6-131I was irradiated with 60Co source with a total absorbed dose of 3.5×106 Rad. There was still no radiolytic deiodination. So CL-6-131I revealed much more thermal and radiolytic stability than CL-19-131I and NCL-6-131I.