The penetration of radioactive CH3131I through adsorbent carbon was studied in air flow systems as a function of bed depth. The count profile in equal increments of depth was found to be exponential with depth along the line of flow for the air-vapor mixtures. The slopes (lognormal count versus depth) were determined for a number of weathered and used carbons as well as for new materials. A large numerical magnitude of the slope is characteristic of new and good carbons; a low value signifies poor retention by the test column. The profile measurements correlate with the percent of penetration. The residual depth profile can serve as an index for the need to replace or to regenerate the carbon bed.