Molecular Sieves are zeolites crystals, made from synthetic alumino-silicates. They are available with different pore sizes. These pore sizes dictate the size and type of molecules that they can absorb. By using different forms of these zeolites, with different pore sizes, you can thus sieve out various molecules based on their molecular diameter. You can think of this process as being exactly the same as the sieving and grading of industrial rocks/aggregates, but just done on a molecular level. At Baltimore, we specialise in 3A Zeolite Powder and Pastes. They can actually remove very small molecules such as water and thus they function as desiccants.