The impact of organic compounds on iodine volatility was investigated under a range of postaccident chemical conditions expected in a reactor containment structure. The rate of production of volatile iodine was evaluated in the presence of 10-3 M concentrations of carbonyl, alkyl halide, and aromatic compounds. A bench-scale apparatus, installed in the irradiation chamber of a Gammacell, was used to measure the rate of iodine volatilization from 10-6 to 10-4 M CsI solutions with pH values from 5 to 9. The results indicated that organic compounds could be classified into groups, based on their distinct effects on iodine volatility. Iodine volatilization increased significantly, up to two orders of magnitude, in the presence of carbonyl compounds and alkyl chlorides, while it decreased in the presence of aromatic compounds. Gas phase speciation indicated that organic iodides dominate the airborne iodine species in the presence of carbonyl compounds and alkyl halides.