AbstractThe paper describes a modification of the Kozeny theory of the air‐permeability method for determining the specific surface of powders to take into account the effect of “slip” at the walls of the pores in the bed of powder. The dependence of the measured value of specific surface on the mean pressure in the bed has been examined experimentally, and it is shown that, by making the correction for “slip,” values of specific surface are obtained substantially independent of the mean pressure (and hence of the mean free path of the gas molecules) in the region of atmospheric pressure. It is also shown that, if the modified equation is used, good agreement is obtained between values of specific surface determined by the “air” method and by a “liquid” method using water, acetone or n‐hexane. A full account is given of the experimental work which has included measurements on six powders ranging in surface area from 0·4 to 5 sq. metre/g.