The enthalpies of formation of selected intermediate phases in the binary systems Ta-S and(, )Ta-Al were determined via thermochemical measurements at(, )>1000(DEGREES)C. The measurements of partial pressures of sulfur species were(, )accomplished by the mass loss variation of the Knudsen effusion method and led to the following (DELTA)H(,f,298)(DEGREES) values: Ta(,2)S: -41.8 kcal/mole, Ta(,6)S: -48.6. Partial pressures of Al over condensed Ta-Al phases were obtained by the coupled mass loss-mass spectrometric version of Knudsen effusion, resulting in enthalpies of formation of: TaAl(,3): -22.7 kcal/mole, Ta(,2)Al(,3): -22.6, Ta(,2)Al: -8.2, Ta(,4)Al: -10.4. Estimated error is (+OR-)0.7 kcal/mole atoms in all cases;Data analysis leading to these values demonstrated agreement with the Neumann-Kopp rule for heat capacities of solid compounds in the Ta-Al system but not for Ta-S phases. In the latter case, a derived set of free energy function values for Ta-S, required for agreement of 2nd law and 3rd law determinations of reaction enthalpies, was assessed as physically reasonable when compared to values for the component elements;Enthalpies of atomization for phases in each system were compared to those of chemically similar phases and were seen to be consistent with composition. Comparisons between the two systems suggest that Ta-S interactions are stronger than Ta-Al interactions in energetic terms, and a tentative explanation is proposed;Evaluations of bond strengths and stabilities in transition metal sulfides and aluminides in general were based on features of the models proposed by Brewer and Engel and by Miedema. The relevance and adequacy of these models are discussed as applied to;the systems in question, and reasons for deviations from predictions by the models are proposed; ('1)DOE Report IS-T-1135. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.