Air-conditioning systems in PWR nuclear power plants employ activated carbon filters to remove radioactive iodine compounds. Although the removal efficiency of radioactive iodine by activated carbon is very high, the total removal efficiency of iodine decreases if there is a bypass leak from a filter frame, gasket or elsewhere when the filters are attached to the unit. Thus, when the filters have been installed, it is necessary to ensure that the leak rate is below the tolerance level. The leak rate is determined by measuring the bypass leak of tracer gas that does not pass through the activated carbon bed. However, use of the conventional chlorofluorocarbon (R112) as tracer gas has now been abolished because of the accompanying risk of ozone depletion, so it is necessary to find an alternative reagent. Here, we report the results of a series of basic experiments to examine the applicability of alternative reagents.