Phase diagrams are essential as a basis for understanding the relationships between stoichiometry, structure and properties of inorganic materials, especially complex materials which have variable composition. In materials based on solid-solution phases, it is necessary to have information on solid-solution mechanisms in order to understand the detailed crystallography and properties of the phases. This is because, in many cases, properties vary dramatically with solid-solution composition. An overview is given of solid-solution equilibria, structures and properties for six ceramic systems: lithium ion conducting solid electrolytes based on Li₄SiO₄ solid solutions, Na⁺ ion-conducting β-aluminas and the effect of Li/Mg additions, lanthanum zirconium tantalate ceramics, doped lithium niobate and lithium tantalate ferroelectrics, bismuth cuprate superconductors and silicon aluminium oxynitride (SIALON) engineering ceramics. As well as providing fundamental data on the stoichiometry and stability of the phases concerned, it is shown how, in each case, valuable clues regarding the likely crystallography of the solid solutions can be obtained.