This book about deals with the thermodynamic assessment of experimental data. In the first three sections of the text an extension is given of the theory on isobaric binary systems. The fourth section gives an account of the thermodynamic analyses of four isobaric binary key systems, highlighting the power of empirical, (exo)thermodynamic correlations. The fifth section is devoted to the thermodynamic description of ternary systems. The last three sections concentrate on the properties of materials, and the phase behaviour of systems under the conditions of high temperature and high pressure, conditions that prevail in the interior of the Earth. A new equation of state is the subject of the sixth section. In the seventh section a move is made to statistical thermodynamics and vibrational models, the description of the systems has changed from mathematical to physical. The last section is on the system MgO - SiO2, looked upon from a geophysical point of view.