A data base is being developed for calculation of quasi-binary and quasi-ternary phase diagrams of ceramic systems (1–3). Previous segments of this base cover combinations of Cr 2O 3, MgO, Al 2O 3, Fe 2O 3, Fe 3O 4, “FeO”, SiO 2, CaO, Si 3N 4 and AlN. Lattice Stability, Solution and Compound Phase Parameters were derived covering the liquid, spinel, corundum, periclase, crystobalite, tridymite, quartz, hexagonal and beta prime phases which appear in the binary systems composed of pairs of these compounds. Compound phases formed from specific binary combinations of these compounds (i.e. MgO·Cr 20 3) were also characterized. This description is based on observed thermochemistry and phase diagrams for the binary systems of interest. Selected ternary systems have been computed based on the foregoing data base for comparison with experimental sections in order to illustrate the usefulness of the data base. The present paper extends the data base to cover BeO, Y 2O 3 and Ce 2O 3 additions. Moreover, ternary sections in the SiO 2-MgO-Si 3N 4, SiO 2-Y 2O 3-Si 3N 4 and SiO 2-Ce 2-O 3-Si 3N 4 were calculated between 1900K and 2100K for comparison with experiment.