Data from the Three Mile Island Unit 2 (TMI-2) accident have shown that only small amounts of iodine and cesium escaped the plant, on the order of tens of curies. To assess the chemical and physical processes responsible for such a limited release, a detailed investigation of iodine and cesium chemical and transport behavior during the core degradation phase of the TMI-2 accident was initiated. Analyses indicate elemental iodine and cesium release from fuel, which subsequently reacts with the high-temperature steam/ hydrogen effluent to produce the gaseous species Csl and CsOH. Partial condensation and chemisorption of CsOH in the upper reactor plenum and on hot-leg piping is also predicted, where loss of the CsOH molecule from a mixture of H2O, Csl, CsOH, and HI gases in chemical equilibrium can result in destabilization of the Csl molecule to replace CsOH, forming HI in the process. Likewise, Csl reaction with borated water is predicted to have resulted in partial conversion of Csl to cesium borate and HI. A combination of Csl, HI, and CsOH is therefore assessed to be the principal form of iodine and cesium transport leaving the reactor vessel during core degradation. These species were subsequently dissolved in water, resulting in large-scale retention of fission product iodine and cesium.