What is a desiccant? The dictionary defines desiccant as a substance that has a high affinity for water and is used as a drying agent such as calcium oxide and silica gel. How does a desiccant act as a drying agent? Desiccant attracts moisture from the air by creating a low vapor pressure at the surface of the desiccant. Since the vapor pressure of the humid air is higher than the vapor pressure of the desiccant, the water molecules move from the air to the desiccant.