The effective diffusivities of HD (Deuterated Hydrogen) and HDO (Deuterated Water) in a Pt/SDBC (Styrene Divinylbenzene Copolymer) catalyst under the conditions of a hydrogen-water isotopic exchange reaction were evaluated. The dynamic-pulse response technique for a helium-nitrogen system was used to determine the tortuosity factor of the SDBC pellet, and the effective diffusivities of HD and HDO were calculated from it under the water vapor conditions. The SDBC single pellet was prepared using the bulk polymerization method; the experimental apparatus consisted of a diffusion cell, a pulse injection valve, a thermal conductivity detector, and the flow control valves. The tortuosity factor of the SDBC single pellet was 2.94. The effective diffusivities for HD were calculated to be 0.0096, 0.010, and 0.0101 cm2/s at 40, 60, and 80 ℃, respectively; they were 0.0040, 0.0041, and 0.0042 cm2/s, respectively for HDO. These values can be used to calculate the Thiele modulus and the effectiveness factors that represent the mass transfer effect in a catalyst pore.