Molecular Sieve type 4A is an alkali alumino silicate; it is the sodium forms of the type A crystal structure.4A molecular sieve has an effective pore opening of about 4 angstroms (0.4nm). 4A molecular sieve will adsorb most molecules with a kinetic diameter of less than 4 angstroms and exclude those larger.

Such adsorbable molecules include simple gas molecules such as oxygen, nitrogen, carbon dioxide and straight chain hydrocarbons. Branched chain hydrocarbons and aromatics are excluded.