Molecular sieve (also called as synthesized zeolite) is a kind of silica-aluminates with micro-porous crystal. It is a frame-work which is constructed by SiO₂ and Al₂O₃.

Type 3A molecular sieves should be used to dry dehydration solvents for electron microscopy. Most common solvents (acetone, ethanol, and methanol, etc.) need to be anhydrous for electron microscopy embedding work using epoxy resins, yet they have a tendency to pick up atmospheric water when bottles are opened. Molecular sieves are used to dehydrate the solvents used in the final stages of dehydration and embedding . Molecular sieves are typically zeolite compounds that strongly adsorb water and have carefully controlled pore sizes.

• 3A (pore size 3 Å): Adsorbs NH₃, H₂O, (not C₂H₆), good for drying polar liquids.
• 4A (pore size 4 Å): Adsorbs H₂O, CO₂, SO₂, H₂S, C₂H₄, C₂H₆, C₃H₆, ethanol. Will not adsorb C₃H₈ and higher hydrocarbons. Good for drying nonpolar liquids and gases.
• 5A (pore size 5 Å): Adsorbs normal (linear) hydrocarbons to n-C₄H₁₀, alcohols to C₄H₉OH, mercaptans to C₄H₉SH. Will not adsorb isocompounds or rings greater than C₄.
• 10X (pore size 8 Å): Adsorbs branched hydrocarbons and aromatics. Useful for drying gases.
• 13X (pore size 10 Å): Adsorbs di-n-butylamine (not tri-n-butylamine). Useful for drying HMPA.