The invention relates to a method and an apparatus (herein referred to as a "gas sorption/desorption analyzer") for measuring the gas sorption properties of substances (for example hydrogen sorption by metal alloys). Measurements include: Pressure Composition Temperature isotherm (PCT), Kinetic, Cycle-life, and density. Measurements are made by sorption of aliquots of gas to or from a sample of the substance. The amount of gas in each aliquot is determined from the gas pressure and temperature in calibrated reservoir volumes. The apparatus comprises components rated for operation up to 200 atm, a plurality of sensors covering a broad pressure range, and minimized volumes to enable accurate measurements of small samples. Aliquot pressures are controlled using a feed-back controlled pressure regulator that can also be used for constant pressure sorption measurements. The gas temperature is regulated using a temperature controlled enclosure. The apparatus also comprises a plurality of safety and failsafe mechanisms.