Two alloys in the Ti-Al system (Ti-45 a/o Al,Ti-62Al) and three alloys in the Ti-Al-Cr system(Ti-47Al-2Cr, Ti-47Al-13Cr, Ti-51Al-12Cr) were selectedfor a thermodynamic study because of interest in their high-temperature oxidation behavior. Activitiesof Al and Ti were measured using a twin Knudsen-cellassembly with one cell acting as an internal standard.For the Ti-45Al alloy, Al activity was also measured with an EMF technique. The Ti-Al-Cr data wereconsistent with the Al and Ti activities expected fromthe adjacent binary Ti-Al phase fields. Implications ofthis work on oxidation properties arediscussed.