[en] The phase stability of a compound is determined by its Gibbs energy, G(T, P)= H - TS. The enthalpy (or enthalpy of formation) is often known from direct measurements or from estimates. The entropy S has been much less studied, and that will be the main theme of this paper. For details, the reader is referred to our recent work, which has dealt with, e.g., 3d-transition metal carbides, nitrides, and oxides (Fernandez Guillermet and Grimvall, 1989a, 1990, 1992); transition metal diborides (Fernandez Guillermet and Grimvall, 1990b); and alkali halides and hydrides (Grimvall and Rosen, 1983; Haglund and Grimvall, 1990), as well as pure transition metals (Fernandez Guillermet and Grimvall, 1989b, c; Grimvall et al., 1987). Here we shall add some material on geologically interesting solids. The outline of the chapter is as follows. In the next section we introduce the concept of an entropy Debye temperature O_s(T) with aluminum silicates chosen as an illustrative example. We also compare, for corundum, the temperature dependence of O_s with that of a conventional Debye temperature derived from the temperature-dependent elastic constants. In the section that follows we show how the atomic masses separate from the interatomic forces in the quantity O_s. This allows a comparison of chemically similar compounds, where the difference in atomic masses is the main reason for the difference in entropy. We also introduce an effective force constant k_s that is derived from the entropy data. We then devote the next section to a study of regularities in k_s and consider two groups of compounds that have the same crystal structure (NaC1) but very different chemical bonding, alkali halides and some transition metal carbides. We end with a brief account of the role of correlations for bonding properties in the modeling and prediction of phase diagrams