A direct method used for measuring the adsorption of water vapor on cellulosic materials and in which a number of improvements have been made is described. It is ensured that no vapors other than water are present no matter how long a time is required for the establishment of equilibrium. The adsorption and desorption isotherms of standard cellulose, spruce wood, surgical cotton, Kodak rag cellulose and bleached sulphite have been measured, and the data which indicate the time required for the establishment of true equilibrium have been given. The data of Urquhart and Williams (6, 7) for standard cellulose determined by an indirect method in 1926 have been shown to be correct.