The final objective of this project is to study the transport properties of radionucleides by geological fluids from the storage section to the surface or to the contact with underground water. the determination of thermodynamic properties at infinite dilution of rare earth ions over wide ranges of temperature (298-573K), pressure (up to 28 Mpa) is then needed to optimize the prevision of their solubility, reactivity and transport properties in geological fluids using predictive models. For that work, triflate anions are preferably used in studying rare earth elements properties as they are one of the only ions that are not leading to the formation of strong complexes in solution. The first step of this study, that will be presented here, is the determination of the thermodynamic properties at infinite dilution of triflate ions over wide ranges of temperature and pressure. We have measured apparent molar volumes and heat capacities for aqueous solutions of trifluoromethane sulfonic acid (triflic acid, HCF3SO3) and sodium trifluoromethane sulfonate (sodium triflate, CF3SO3Na) up to 573 K and 28 Mpa in order to determine the apparent molar volumes and heat capacities of triflate ion at infinite dilution. Apparent molar volumes were obtained using a high temperature (Pt/Rh) vibrating tube flow densimeter while Apparent molar heat capacities are obtained from a differential flow heat capacity calorimeter. Both apparatuses were built in the laboratory in Clermont-Ferrand. Data were then correlated using Pitzer equations for electrolyte solutions to determine infinite dilution properties and our results were compared to those obtained by Xiao et al.