This Ph.D. project can be divided into two main sections. The first one, covering chapters 2 to 7, deals with the calculation of vapour-liquid, solid-liquid, and speciation equilibria for sparingly soluble salts found in natural waters, under hydrothermal conditions (up to 300ºC and 1000 bar). Chapters 8 and 9 focus on the experimental part of this dissertation, analyzing different experimental procedures to determine salt solubility at high temperature and pressure, and developing a setup to perform those measurements. The motivation behind both parts of the Ph.D. project is the problem of scale formation found in many industrial processes, and especially in oilfield and geothermal operations. We want to contribute to the study of this problem by releasing a simple and accurate thermodynamic model capable of calculating the behaviour of scaling minerals, covering a wide range of temperature and pressure. Reliable experimental solubility measurements under conditions similar to those found in reality will help the development of strong and consistent models. Chapter 1 is a short introduction to the problem of scale formation, the model chosen to study it, and the experiments performed. Chapter 2 is focused on thermodynamics of the systems studied and on the calculation of vapour-liquid, solid-liquid, and speciation equilibria. The effects of both temperature and pressure on the solubility are addressed, and explanation of the model calculations is also given. Chapter 3 presents the thermodynamic model used in this Ph.D. project. A review of alternative activity coefficient models an earlier work on scale formation is provided. A guideline to the parameter estimation procedure and the number of parameters estimated in the present work are also described. The prediction of solid-liquid equilibrium of sulphate scaling minerals (SrSO4, BaSO4, CaSO4 and CaSO4•2H2O) at temperatures up to 300ºC and pressures up to 1000 bar is described in chapter 4. Results for the binary systems (M2+, )-H2O; the ternary systems (Na+, M2+, )-H2O, and (Na+, M2+, Cl-)-H2O; and the quaternary systems (Na+, M2+)(Cl-, )-H2O, are presented. M2+ stands for Ba2+, Ca2+, or Sr2+. Chapter 5 is devoted to the correlation and prediction of vapour-liquid-solid equilibria for different carbonate systems causing scale problems (CaCO3, BaCO3, SrCO3, and MgCO3), covering the temperature range from 0 to 250ºC and pressures up to 1000 bar. The solubility of CO2 in pure water, and the solubility of CO2 in solutions of different salts (NaCl and Na2SO4) have also been correlated. Results for the binary systems MCO3-H2O, and CO2-H2O; the ternary systems MCO3-CO2-H2O, CO2-NaCl-H2O, and CO2-Na2SO4-H2O; and the quaternary system CO2-NaCl-Na2SO4-H2O are given. M2+ stands for Ca2+, Mg2+, Ba2+, and Sr2+. This chapter also includes an analysis of the CaCO3-MgCO3-CO2-H2O system. Chapter 6 deals with the system NaCl-H2O. Available data for that system at high temperatures and/or pressures are addressed, and sodium chloride solubility calculations are performed up to 2000 bar. Chapter 7 includes a validation of the model by comparing our predictions to data for real natural waters not used during the parameterization. A very good agreement between our predictions and the field observations is obtained, making us confident about the validity of the model, despite the high temperature, pressure and ionic strength of the tested systems. In chapter 8, alternative experimental procedures to determine solubility at high temperature and pressure are introduced: Synthetic fluid inclusion technique, electrochemical technique, quartz crystal microbalances, and conductivity measurements. Chapter 9 describes the experimental procedure followed and the setup employed to determine barite solubility in NaCl solutions at high temperature and pressure. The analytical technique chosen to determine Ba2+ concentrations (inductively coupled plasma mass spectrometry) is explained. Few experimental results for the systems NaCl-H2O (used for validation) and BaSO4-NaCl-H2O are reported and analyzed. Chapter 10 contains the conclusions of the Ph.D. project.