The theme of this thesis is the high-temperature phase studies of inorganic metal sulphates and the aspects specifically dealt with are phase diagrams and drop calorimetry. Chapters 1,2, and 3 are concerned with the first of these. Chapter 1 briefly describes the features encountered in one and two-component systems and provides the thermodynamic background which allows such systems to be calculated. Chapter 2 describes a selection of experimental techniques used to determine phase diagrams. Apparatus to perform Differential Thermal Analysis (DTA) was constructed as part of the work of this thesis and so this technique is fully described, including details of the apparatus, and a section on interpretation of DTA results relating to phase studies. Pointers are included as to further applications of this technique and improvements or modifications that might be made to the present apparatus. Chapter 3 presents the phase diagram of the previously undetermined system Ag2SO4-LA2(SO4)3 and also includes some observations on the kinetics of some phase changes and a description of the optical furnace which was built to study them. Also presented is a re-investigation of the system Ag2SO4-CdSO4, which contains many features not evident in an earlier study. Chapter 4 is concerned with the thermodynamics of substances at high-temperatures and describes in detail the design, construction and experimental procedure for a drop calorimeter. This apparatus allows the equilibrium temperature of high-temperature polymorphic phase transitions and their enthalpies of transition to be determined. A preliminary study on CdSO4 was performed, which ties in with the study in Chapter 3, and an attempt was made to resolve the long standing controversy on the temperatures of the CdSO4 phase transitions. Drop Calorimetry can provide some of the data necessary for the calculation of phase diagrams as described in Chapter 1, and so a brief introduction to the thermodynamics of non-reacting systems and the treatment of results is presented.