The flow techniques used for measuring calorimetric and volumetric properties of liquid mixtures at room temperature were introduced by P. Picker at the beginning of the seventies. The results obtained using those techniques are of major interest in the study of aqueous solutions where the knowledge of thermodynamic properties of solutes at infinite dilution is needed [1]. Regarding aqueous solutions at high temperature and pressure, theoretical work done at Berkeley around 1980 by K. Pitzer [2] (theory of ionic interactions) and HC Helgeson [3] (HKF model) have strongly influenced the modeling of hydrothermal systems. They highlighted the usefulness of calculating activity coefficients by integrating derived thermodynamic properties of solutes (volume, enthalpy, heat capacity) available from calorimetric and densimetric measurements. The development of new devices then became a necessity and a challenge. RH Wood at the University of Delaware has been the pioneer in the implementation of flow techniques at super-ambient conditions [4] and his group made the first measurements of heat capacities and volumes of dilute solutions near to the critical point of water. After the "American period", more systematic work followed in the 90's trying to elucidate and quantify the behavior of aqueous solutions at HT/Hp conditions. Two instruments, unique in Europe, have been developed in Clermont-Ferrand in order to get derivative properties up to the critical point of water (T <700 K, p <40 Mpa) [5,6]. These devices will be described here in the context of experiments with aqueous systems containing electrolyte or non-electrolyte molecules. Examples will include volatile solutes, weak acids, lanthanides in water. Some results will be shown also concerning dissolution of gas (CO2) in aqueous solutions [7-9].