A handful of the available molecular sieve materials are utilized in isotope work. Molecular sieve or trapping processes are rarely used in combination. A small pore sieve could be used to separate small molecules from large ones. A large pore sieve would then be used to separate the larger gas species according to boiling point. The molecular sieves are used to improve vacuum performance, collect gases in the field, separate gases, and transfer gases within vacuum systems. The isotopic compositions of the gas of interest remain intact at the completion of the operation, except for the first of these processes. The molecular sieves such as zeolites are not more widely used considering that they are very sturdy and non-toxic substances. High-precision stable isotope analysis typically requires a good vacuum in preparation systems and a clean working gas in the mass spectrometer. Future studies are needed to investigate the use of other molecular sieves such as the high silica zeolites and non-zeolitic materials.