Measurements have been made of the equilibrium permeabilities of softwoods to air of different relative vapor pressures. Transverse sections less than the average fiber length in thickness, in which the open cavities account for practically all of the permeability, show practically no change in permeability with changes in the equilibrium relative vapor pressure. Sections thicker than the maximum fiber length, the permeabilities of which are dependent upon the size of the pit openings, show an increase in permeability with a decrease in equilibrium relative vapor pressure. When the square root of the permeability is plotted against the moisture content of the wood in equilibrium with the various relative vapor pressures of air, practically a linear relationship is obtained from 0 to 20 percent moisture content. At higher moisture contents the permeabilities are considerably less than the linear relationship calls for. This is due to films of water forming across the capillaries. Higher pressures than those used are required to overcome the effect of the surface tension of the water in these capillaries. A new means of determining the distribution of size of openings in a porous membrane based on these findings is given.