The thermal diffusion in H_2-CO_2 system has been measured at pressures up to 120 atm and a mean temperature of 339.6～346.0°K. A "two-bulb" type apparatus has been employed here, because accurate estimation of thermal diffusion factor is possible. The thermal diffusion cells used are made of glass at 1～11.5 atm, and of steel at higher pressures. The composition at a steady state is analysed chemically. The experimental results obtained are as follows: 1) At 0.5～3.5 atm, the effect of pressure upon the thermal diffusion factor α is very small. and α is nearly constant as predicted by the kinetic theory. 2) The change in α with the composition is examined at the moderate pressure range of 1～11.5 atm, and it is found that the maximum separation is obtained near 0.6 of mole fraction of H_2, and that α increases with increasing mole fraction of H_2. 3) The theoretical equation of thermal diffusion at the atmospheric pressure is only qualitative for this system. 4) At pressures up to 120 atm, α increases continuously with increasing pressure, and the present results are found to have a good agreement with Becker's up to 81 atm
