Abstract: A method and apparatus for measuring physical adsorption of a gas by a solid adsorbent are disclosed. Use is made of a capacitance cell in controlled gas flow communication with a sample cell initially under vacuum and containing the solid adsorbent. The capacitance cell is filled with an adsorptive gas at a predetermined pressure and the dielectric constant of the gas at the predetermined pressure is measured to determine a first density value. The gas from the capacitance cell is then allowed to expand into the sample cell for adsorption by the solid adsorbent, whereby an adsorption takes place pressure falls until eqilibrium is established. The dielectric constant of the gas at equilibrium pressure is measured to determine a second density value, and the amount of gas adsorbed by the solid adsorbent is determined from the first and second density values.