This document specifies a test method using radioactive methyl iodide (CH3131I) as a tracer to determine the in-situ decontamination factor of an iodine trap. An in-situ test allows to reach the global efficiency of the trap characterized by the sorbent efficiency but also by the implementation of the trap within the ventilation duct) while the intrinsic efficiency of a charcoal is characterized in a laboratory by ISO 18417[9] (or other national standards such as ASTM D3803[10]). This document provides general and common requirements for this method to assess the efficiency of an iodine trap, but also, the tools requirements, accuracy and the provisions needed to ensure safety of the workers, public and the environment during the test. This reproducible method can support nuclear facility operators as a reference method to compare the decontamination factor evaluated by this method to reference values (e.g. safety criteria, national legislation, etc.). Because of the use of a radioactive tracer, some precautions should be applied. Firstly, this method is usually used for ventilation systems with monitoring of gaseous iodine releases in environment in accordance with the national regulations. Secondly, this method is not used to determine the decontamination factor of iodine traps used in ventilation systems with air release in rooms with potential presence of workers (e.g. control room). For those rooms, a non-radioactive method is preferred. This document can apply to installations with low inventory of radioiodine equipped with iodine traps (e.g. small laboratories). In this case, some provisions can be adapted but always in accordance with the national regulations. Finally, this document mainly deals with iodine traps using impregnated activated carbon. However, this method can be used with some adaptations to other solid sorbent as inorganic sorbent (e.g. zeoli