For the first time, using aqueous solution calorimetry, we clearly identify the chemisorption of an unusually strong iodine charge-transfer (CT) complex within the cages of a metal organic framework. Specifically, we studied the sorption of iodine gas in zeolitic imidazolate framework-8 (ZIF-8, Zn(2-methyl-imidazolate)(2)). Two iodine-loaded ZIF-8 samples were examined. The first, before thermal treatment, contained 0.17 I-2/Zn on the surface and 0.59 I-2/Zn inside the cage. The second sample was thermally treated, leaving only cage-confined iodine, 0.59 I-2/Zn. The energetics of iodine confinement per I-2 (relative to solid I-2) in ZIF-8 are Delta H-ads = -41.47 +/- 2.03 kJ/(mol I-2) within the cage and Delta H-ads = -18.06 +/- 0.62 kJ/(mol I-2) for surface bound iodine. The cage-confined iodine exhibits a 3-fold increase in binding energy over CT complexes on various organic adsorbents, which show only moderate exothermic heats of binding, from -5 to -15 kJ/(mol I-2). The ZIF-8 cage geometry allows each iodine atom to form two CT complexes between opposing 2-methylimidazolate linkers, creating the ideal binding site to maximize iodine retention.