Refluxing capsule tests were conducted to evaluate the corrosion properties of alloys Nb, Nb-1% Zr, Nb-10% W-1% Zr, Ta-8% W-2% Hf, Mo-0.5% Ti-0.1% Zr, Nb-10% W-10% Hf-0.5% Y, and W-26% Re in boiling potassium at 1100 to 1400°C. Nb-1% Zr was also evaluated in refluxing sodium, rubidium, and cesium at 1200°C. Alloys of niobium or tantalum that contained strong oxide formers, such as zirconium or hafnium, showed negligible corrosion effects up to 1300°C for times as long as 5000 h. Nb-1% Zr exposed to the other alkali metals was similar. Weight changes in insert specimens from condenser sections were small and in most tests were traced to the migration of oxygen in the system. Unalloyed niobium, on the other hand, was very heavily attacked under similar conditions. Mass transfer of tungsten was observed in a W-26% Re capsule. Capsule geometry can be an important variable in comparing the results between various corrosion investigations.