Deactivation of a hydrophobic Pt/SDBC catalyst for the H2/HTO isotopic exchange reaction used to remove tritium from the waste water generated in a nuclear-fuel reprocessing plant has been studied experimentally. The catalyst was poisoned reversibly by a small amount of HN03 and could be regenerated by washing with water followed by drying in an inert gas. As a countermeasure against this poisoning, the neutralization of the waste water was found to be effective. The presence of I2 in the waste water caused a sharp decrease in the activity of the catalyst, due to the irreversible adsorption of I2 onto the catalyst surface. The I2 poisoning could be prevented by the conversion of I2 into I− or IO3− by neutralization or redox reaction. TBP and the neutral nitrate salts of fission products such as Sr(NO3)2 showed negligible poisoning effects on the catalyst.