Mass transfer rates of Nb-1%Zr and D-43 (Nb-10%W-1%Zr-0.1%C) in high-temperature lithium were determined in thermal convection loops of each alloy. The loops circulated lithium for 3000h at a 1200°C maximum temperature and a 100 to 150°C ΔT. Weight change data were obtained from 58 insert specimens placed end-to-end around each loop. The oxygen content of both alloys decreased during test. Zirconium and nitrogen were transferred by the lithium from the higher-temperature to the lower-temperature loop surfaces. The extent of this mass transfer was greater in the Nb-1%Zr test and resulted in a light zirconium-nitride mass transfer deposit on the cold-leg surfaces of this alloy. Specimens at equivalent temperatures in the heater and cooler regions showed the same weight change indicating no appreciable downstream effect due to concentration changes in the lithium. Given the same absolute concentration driving force, the rate of deposition was measurably greater than the rate of dissolution; accordingly, dissolution occurred over two-thirds of the loop surfaces and deposition over the remaining one-third. An analysis of the mass transfer results for these two loops based on the assumption of a simple dissolution model showed the change in solute concentration of the lithium around the loop to be only a few parts per billion.