The high temperature water retention of new composites with PFSA (polyfluoro sulfonic acid) polymer matrix is investigated based on structural analysis of such polymers containing a mineral acid and MgF2-nanoparticles as additives. As compared to the parent polymer the temperature for water removal is increased by almost 50°C to 250°C. The thermal stability of the PFSA polymer is slightly decreased from 360 to 340°C with respect to the onset temperature of de-sulfonation and back-bone de-polymerization. Different PFSA polymer composite membranes with a fixed amount of sulfuric acid as additive and a varying amount of 3 to 15 weight % of MgF2-nanoparticles coated glass particles were investigated. Structural details of the PFSA evident from FTIR-ATR analysis of the solid membranes can be associated with two major phenomena: (1) dissociation of the proton from the sulfonic acid group in the presence of small amount of water and (2) changes in the conformation and / or the degree of the crystallinity of the PTFE hydrophobic domains composite membranes in the PFSA as a function of the polymer and the weight ratio of the solid additive. From these results evidence is given that sulfuric acid acts as bridging agent to facilitate direct bonding between the sulfonic acid groups of the PFSA polymer and MgF2-nanoparticles, which are deposited as coating on the surface of micron-size glass particles used for mechanical strengthening of the polymer. The significantly increased water retention capability is attributed to the intimate bonding between the PFSA polymer and the solid inorganic additive. References: (1) Di Noto V.; Gliubizzi, R.; Negro, E.; Pace, G. J. Phys. Chem. B 110, 2006, 24972. (2) Di Noto V.; Gliubizzi, R.; Negro, E.; Vittadello, M.; Pace, G. Electrochim. Acta 53, 2007, 1618. (3) Di Noto V.; Piga, M.; Piga, L.; Polizzi, S.; Negro, E. J. Power Sources 178, 2008, 561. (4) Di Noto, V.; Piga, M.; Negro, E.; Giffin, G. A.; Polizzi, S.; Zawodzinski, T. A. RSC, Adv., 2013, 3, 18960. (5) Di Noto, V.; Piga, M.; Giffin, G. A.; Keti, V.; Zawodzinski, T. A. J. Am. Chem. Soc. 134, 2012, 19099.