[en] The method of detecting predetermined organic halogen compounds in a gas comprising the steps of directing a sample of said gas into a reaction chamber containing immobilized therein a halogen compound in which the halogen constituent is radioactive, providing environmental conditions in said chamber to promote a chemical reaction whereby said radioactive constituent forms a mobile compound with a constituent of said organic halogen compound, and passing said sample including any of said mobile compound generated out of said reaction chamber and into the presence of a radiation detector