Dynamic behavior of cryogenic distillation columns has been studied with an H-D-T system (1.5 g of tritium) under single column and two-column cascade operation modes. The columns used were different in inner diameters (1 cm and 2 cm) and in size packings (3 mm Dixon rings for the larger column and 1.5 mm Dixon rings for the smaller column). For both of the columns, the experimental observations for composition distribution at the steady-state was in close agreement with calculated results for all the components. The HETP values measured were in the range from 3 to 6 cm. The dynamic variations of compositions in bottom product streams experimentally observed were well predicted by computer-aided simulation. For top product streams, the variations were significantly slow in comparison with those of the calculated results using an assumption that liquid holdups in condensers were negligible. These results indicate that the columns have appreciable liquid holdups at the condensers.