The thermochemical water-splitting iodine-sulfur (IS) process requires corrosion-resistant materials owing to usage of corrosive fluids, such as a mixture of HI-I-2-H2O. Fluoropolymers, such as PTFE and PFA, are adaptable as lining materials for protecting plant components. However, there has been a concern: PTFE and PFA have the ability to permeate various permeants. From the viewpoint of corrosion of the base material, the permeation characteristics of HI and I-2 should be evaluated to improve the integrity of the IS process. In this study, permeation tests on PTFE and PFA membranes were performed to measure the permeated fluxes of HI and I-2, and the effects of the operating conditions on them were investigated. The introduction of a permeability parameter could be successful for normalizing the permeated fluxes for a specific membrane thickness and a vapor pressure. Then, the empirical formula of the permeability was given as an Arrhenius-type equation to use as a plant design. Finally, based on the results, the proper conditions for design of a lining material for the inhibition of HI and I-2 permeation are summarized. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.