Changes with time in the isotopic exchange reaction rate between H-2 and HDO(nu) over a hydrophobic Pt/SDBC (styrene-divinylbenzene copolymer) catalyst induced by iodine have been studied experimentally. The addition of a trace amount of I-2 to the reactor caused a remarkable decrease in the activity of the catalyst. The measurement results by means of the gas chromatographic pulse technique show that the H-2 adsorption onto the catalyst surface was strongly inhibited by iodine. A kinetic model for the poisoning was developed in terms of the competitive adsorption of I-2 with H-2 onto the catalytic active site. The experimental results were explained reasonably by the proposed kinetic model based on the site balance by assuming that I-2 adsorbs non-dissociatively on the active site. The poisoning could be prevented by the conversion of I-2 to I- or IO3- through redox reactions.