The thermodynamic activities of Fe and Cr in the austenitic phase of a D-9 alloy were dtermined by a metastable emf method, employing single crystal CaF2 as the electrolyte. The activities of these metallic components are compared with those in the austenitic phases of AISI types 304, 316 and 316 LN stainless steels, reported earlier in the literature. The values of these activities were made use of, in computing threshold oxygen levels in liquid sodium for the formation of corrosive products, NaCrO2 and Na4FeO3 as a result o interaction of these structural alloys in LMFBRs. By a similar exercise, the application of these activity data in the computation of threshold tellurium potentials for the formation of Cr7Te8 as a result of fission product-clad chemical interaction, and of uranium potentials for the formation of UFe2, as a consequence of fuel-clad interaction, have also been demonstrated.