The effect of outdoor weathering of impregnated charcoal beds used for the trapping and retention of methyl 127I vapor was studied under various dynamic flow conditions. The data showed that the first or inlet segment of the bed suffered the greatest degradation in its role as a guard layer for the remaining segments of the bed. Test results were analyzed in terms of the parameters of adsorption capacity and adsorption and desorption rate constants. As was expected, the desorption rate constant was much smaller than the adsorption rate constant. The dimensionless ratio of the adsorption to the desorption rate constant can be considered as a retentivity index for any particular charcoal.