The separation factors of a cryogenic-wall thermal diffusion column have been measured with H-D and H-T systems. The column was 1.5 m in height and 0.03 m in diameter. Two types of heaters were tested: a tungsten wire 0.5 mm in diameter and a stainless steel sheath heater 11 mm in diameter. The maximum separation factors using the tungsten wire were 49 for an H-D system and 284 for an H-T system under the total reflux mode at 1273 K. At the feed flow rate of 10 cm3/min, the separation factor using the tungsten wire was 55 for the H-T system at 1273 K. The separation factor was decreased as the diameter of the heater was decreased; and the optimum pressure was increased with the diameter of the heater. In the case where the sheath heater (11 mm) was used at 10 cm3/min with the H-T system, the maximum separation factor reached 2660 even at 763 K.