A nondestructive in-place test procedure was developed to measure the efficiency for iodine removal by charcoal filters in the Hanford reactors exhaust gas systems. Tests were performed during the past few months on the C, KE, and KW Hanford single pass production reactor systems. Results from tests conducted on individual reactor confinement cells showed iodine trapping efficiencies of from 94 to 98%. The experimental techniques and procedures used also facilitated the simultaneous measurement of iodine plateout in the reactor exhaust duct systems.