A sample is disposed in a sample chamber and is connected by a flow restrictor or "leak" to a bulb of known volume. In adsorption studies, an on/off valve is opened, allowing gas to flow from the bulb into the sample chamber. The leak controls the rate of flow to be less than the equilibrium rate of adsorption/desorption of gas with respect to the sample, such that the pressure in the sample chamber at all times is an equilibrium value. The pressure in the bulb and in the sample chamber are both measured repetitively. The change in pressure in the bulb provides an indication of the net amount of gas admitted to the sample chamber. This quantity together with the change in pressure in the sample chamber can be used to provide an indication of the amount of gas which is adsorbed onto the sample. In desorption studies the bulb is evacuated relative to the sample chamber. The on/off valve is opened to allow gas to flow from the sample chamber into the bulb at a rate controlled by the leak to be less than the equilibrium rate of adsorption/desorption such that the pressure within the sample chamber is at all times an equilibrium value. Points on the adsorption and desorption isotherms can thus be determined continually.