Abstract: The equilibrium relative humidity over a substance may be determined by the following method. The sample of the substance is placed in a lower compartment of a diffusion chamber and a dried gas is then passed through an upper compartment of the chamber which is partitioned from the lower by a disc of water vapor permeable non-hygroscopic material. When the rate of diffusion of water vapor through the disc from the lower to the upper compartment becomes constant the equilibrium relative humidity of the sample is directly related to the moisture concentration in the sweep gas and can be readily determined from a suitable calibration graph for the disc.