The emergency exhaust system of the Pakistan Research Reactor-1 (PARR-1) in corporates charcoal bed filters for the removal of radioiodine from exhaust gases in case of an emergency involving the release of radioiodine. The filters adsorb radioiodine. Testing of the filters is necessary so as to assess their iodine retention efficiency, if they are to meet regulatory requirements. The retention efficiency of our filters has been measured and found to be 99.1%. This value is higher than the value of the activated charcoal filter efficiency (99%) employed to determine the source term for release through emergency exhaust with filtration, for dose calculations. In this paper, the procedure for testing the efficiency of charcoal filters has been discussed and results obtained presented.