The absolute specific volume of cellulose has been determined by means of helium gas. A new experimental procedure has been developed, which, it is claimed, is more accurate than any previously used where a small quantity of porous material is involved. The method is extended to include the measurement of the volume of a system resulting from the adsorption of a vapor on a porous material, and applied in this instance to the system cellulose-water vapor. The specific volume of cellulose in helium was found to be 0.640 ± 0.001 cc. The volume of the system cellulose-water vapor was found to be much smaller than that given by the components cellulose and condensed water, as long as the amount of adsorbed water was below 4%. With more than 8% of adsorbed water the density of subsequently sorbed water was found to be the same as that of the normal liquid.The hypothesis is put forward that the first water adsorbed on cellulose enters into a definite chemical combination, considerable contraction taking place, and that subsequently adsorbed water first causes an increase in cellulose surface, with further consequent contraction in the volume of the two components and then a filling up of the so-called capillary spaces in which the water maintains its liquid properties. The relation of this hypothesis to the hysteresis effect is indicated, the trend which further work should take is pointed out, and the interest of this problem from a practical point of view is discussed.