Abstract A rather straightforward synthetic procedure for the production of new materials is the substitution of principal ions in the crystal for one another, and some general principles relating to structure have been developed which guide the material designer as to which substitutions are likely to lead to stable materials. These are briefly reported. We here consider the thermochemical consequences, rather than the structural implications, of unit-charged cation substitutions, namely of NH 4 + , Li + , Na + , K + , Rb + , Cs + , Tl + , Ag + , and Cu + , among a large group of ionic solids. It is observed that the formation enthalpies hardly differ among these materials, while their absolute entropies, heat capacities, and formula unit volumes follow similar linear trends but with some relative displacements.