The chemical reactions of (i) oxides with each other, e.g. CaO with SiO₂, (ii) oxides with oxysalts, e.g. CaO with CaSiO₃, and (iii) oxysalts with each other, e.g. Ca₂SiO₄ with Ca₃(PO₄)₂, have been considered from the point of view of acid–base theory. From known optical basicity values for alkali and alkaline-earth metal oxides and also for MgO, Al₂O₃, B₂O₃, SiO₂, P₂O₅, and SO₃, optical basicities were calculated for a large number of oxysalts. Examination of existing data on the reactions of these oxidic compounds at elevated temperatures, in the form of sub-solidus compatibility diagrams for ternary systems, has revealed that generally the optical basicity spanned by the products is less than that spanned by the reactants. This indicates that acid–base neutralisation is the major driving force behind the reaction; occasionally this principle is violated, as when other factors such as a stable network structure are of prime importance. It is shown how optical basicity analysis offers a method of constructing the tie-lines of sub-solidus compatibility diagrams, and it is suggested that this might be used as a guide for checking uncertainties in such diagrams.