The tendency of all solid surfaces to attract surrounding gas molecules gives rise to a process called gas sorption. Monitoring the gas sorption process provides a wealth of useful information about the characteristics of solids such as surface area and pore size.

Before performing a surface area analysis or pore size measurement, solid surfaces must be freed from contaminants such as water and oils. Surface cleaning (degassing) is most often carried out by placing a sample of the solid in a glass cell and heating it under a vacuum, or a flow of dry, inert gas.

Once clean, the sample is brought to a constant temperature by means of an external bath, typically a dewar flask containing a cryogen like liquid nitrogen. Then, small amounts of a gas (the absorbate) are admitted in steps into the evacuated sample chamber.

Absorbate molecules quickly find their way to the surface of every pore in the solid (the adsorbent). These molecules can either bounce off or stick to the surface. Gas molecules that stick to the surface are said to be adsorbed. The strength with which adsorbed molecules interact with the surface determines if the adsorption process is to be considered physical (weak) or chemical (strong) in nature.