A substance used in a gas-dehydration unit to remove water and moisture. The desiccant can be liquid, such as methanol, glycol (ethylene, diethylene, triethylene, and tetraethylene). Dessicants also can be solid, such as silica gel or calcium chloride (CaCl₂).

The most common gas-dehydration system (glycol dehydrator) uses liquid desiccants such as diethylene, triethylene and tetraethylene, which are substances that can be regenerated. Regeneration means that the water absorbed by these substances can be separated from them. Some liquid desiccants such as methanol or ethylene cannot be regenerated.

Solid desiccants are also used for gas dehydration. They are placed as beds through which wet gas is passed. The main limitation of the use of solid dessicants is that they absorb only limited quantities of water. When the desiccant saturation point is reached, the solid dessicant must be replaced. Another limitation is that sometimes water cannot be removed from it.