A gas chromatograph is used to measure the amount of nitrogen adsorbed on the samples from which the surface areas are computed. The results are compared to surface areas calculated from the Kozeny-Carman equation using Carman's textural factor of 5. 0. Excellent agreement is obtained for spherical glass beads and crushed sand. However, the ratio of the surface areas of five homogeneous sandstones obtained by nitrogen adsorption to that determined by the Kozeny-Carman equation ranged from 26 to 43. Gas adsorption should be used for surface area measurements of geologic materials.