A method for separating hydrogen isotope species by gas chromatography is described. The invention utilizes low temperature chromatography on molecular sieves with peak cutting or temperature programming depending on the particular application. The method provides a much larger throughput capability than was heretofore available. The invention also includes the separation of mixed dimer isotope species from the helium carrier gas by condensation, and storage thereof at low temperature in a non-catalytic environment so that equilibration may be avoided.