Data are presented on carbon and nitrogen transfer by potassium in 16 stainless-steel thermal convection loops containing Cb-1%Zr and stainless-steel tabs. Typical test conditions were 5000 h at 1600°F maximum, 1200°F minimum. Changes in weight, composition, and microstructure of both tab materials were determined as a function of time over the full temperature range. Changes in loop wall composition, microstructure, and tensile properties were determined at the end of the test. The Cb-1%Zr formed thin carbide and nitride surface films at a rate limited by diffusion through the films rather than by availability of carbon and nitrogen in the potassium. Cold bend tests showed good ductility in Cb-1%Zr after 5000 h. Transfer of carbon and nitrogen from hot to cold stainless steel was unaffected by the Cb-1%Zr and had less effect on tensile properties than microstructural changes associated with the thermal history.