The trapping or retention efficiency of impregnated charcoal beds for the vapor of methylradioiodide tagged with 131I was determined under various flow conditions by radioactive counting of each of eight equal-volume segments of the bed, including that of the large backup section. The trapping was catalytic in nature and obeyed first-order kinetics. Over a ten-fold change in residence time, the rate constant increased nonlinearly with increase in superficial linear velocity in accordance with theory.