A continuous countercurrent process for separation of isotopes of the light elements in gas-solid systems is reported. To separate hydrogen isotopes a hydride-forming palladium adsorbent and a zeolite 13A were used as a solid phase. The molecular sieve was also used to separate oxygen, nitrogen, and argon isotopes. The simulated moving-bed technique (SMB) was employed to organize a counter-currency between gas and solid phases without actual movement of the latter phase. Details of the experimental SMB unit operating in the gas phase are presented, and the results of the separation tests are discussed. A multichannel disk rotary valve was used to switch the flows. It is shown that the process takes advantage of a continuous operation and possesses very high separation efficiency with HETP values comparable to those for the equivalent conventional chromatographic process. It appears that the process proposed is ideally suited for processing tritium-containing streams to produce commodity-grade tritium and may be effectively used to separate isotopes of other light elements.