The synthesis and postgrowth treatment of luminescent materials frequently involve solid-vapor interactions. Because the underlying multispecies, multireaction equilibria are rather complex, quantitative correlations between synthesis conditions, vacancy concentrations, and emissive properties for predictive purposes are not readily available. In order to support the development of predictive guidelines, a quantitative, computational analysis of the vapor phase over ZnS and SrS in the absence and presence of nonconstituent species has been performed. The results confirm the complexity of these systems and show quantitatively the effects of impurity gases on the critical metal to nonmetal ratio in the ambient atmosphere. The data also reveal the temperature ranges in which desirable MIX ratios are available for given experimental conditions. The combined results of such computations are useful as reference data for the optimization and definition of reproducible synthesis conditions, and for the estimation of the type and magnitude of vacancies. (c) 2006 American Vacuum Society.