A molecular sieve is a material with very small holes of precise and uniform size. These holes are small enough to block large molecules while allowing small molecules to pass. Many molecular sieves are used as desiccants. Some examples include activated charcoal and silica gel.

The diameter of a molecular sieve is measured in Angstroms (Å) or nanometres (nm). The unit conversion for 1 Å is 0.1 nm. According to IUPAC notation, microporous materials have pore diameters of less than 2 nm (20 Å) and macroporous materialshave pore diameters of greater than 50 nm (500 Å); the mesoporous category thus lies in the middle with pore diameters between 2 and 50 nm (20-500 Å).