Iodine species released into a reactor containment building following a loss-of-coolant accident is determined by the relative timing and quantity of iodine and other fission products released from the fuel, chemical thermodynamics in the fuel channel, and reaction kinetics in cooler regions of the heat transport system (HTS). Iodine speciation along the transport path from the fuel to cooler regions of the HTS and into containment is evaluated using chemical thermodynamics calculations, leading to a prediction of the volatile iodine mole fraction that theoretically would enter containment. Sensitivities to a decrease in the cesium-to-iodine ratio, a decrease in iodine concentration in the coolant, and an increase in oxygen partial pressure are tested. The role of the presence of other elements, namely, molybdenum, tellurium, uranium, and lithium, are also evaluated. Under most conditions, the mole fraction of iodine entering containment in volatile form is found to be <0.1%. There are circumstances, however, when cesium iodide can be destabilized by a low cesium-to-molybdenum ratio in an oxidizing atmosphere such as steam. To further explore this situation and to validate the code, chemical equilibrium calculations are also compared to earlier Knudsen-cell experimental studies of the interaction of cesium, iodine, molybdenum, and urania. In these experiments, the partial pressures of cesium molybdate and elemental iodine are measured as a function of temperature over the range 1100 to 1500 K. The calculated Cs2MoO4 vapor pressures agree with the experimental results within an order of magnitude at temperatures up to 1200 K; and between 770 and 1150 K, the agreement is within a factor of 2 to 5 depending on the chemical system.