The iodine adsorption characteristics of activated charcoal from the gas confinement systems of nuclear reactors at Hanford were determined. Results from tests conducted under ambient conditions with molecular iodine showed that all reactor charcoals were adequate for continued use. However, at high humidity and temperature, all reactor charcoals exhibited reduction in iodine adsorption of at least 2% and in a jew cases the loss was severe enough to warrant charcoal replacement. Under the more severe adsorption conditions, the iodine efficiency range for the nine reactors tested was 82 to 98%.