The temperature and pressure dependencies of excess Gibbs free energy of mixing functions in the CaAl2Si2O8-KAlSi3O8 binary system have been estimated semiempirically using published Margules parameters for the CaAl2Si2O8-NaAlSi3O8 and NaAlSi3O8-KAlSi3O8 binary systems and compositional data on coexisting experimental feldspars. The calculated anorthite-orthoclase interaction parameters permit simultaneous pressure-temperature solution of activity-composition relations based on anorthite, albite, and orthoclase exchange reactions between coexisting ternary feldspars. The proposed “ternary-feldspar thermobarometer” has been used to calculate equilibration conditions of feldspars in rhyolitic and trachytic rocks. Predicted temperatures of crystallization of feldspar phenocrysts from the Bishop Tuff agree closely with those predicted by Fe-Ti oxide thermometry; calculated pressures of feldspar equilibration are similar to estimates based on Fe-Ti oxide temperatures and the pressure dependence of the albite exchange reaction. The thermobarometer is sensitive to analytical uncertainties in the Or content of plagioclase and the An content of alkali feldspar, and to any systematic errors in the binary mixing parameters.