The relative difference (Δb/b) between the electromigration mobilities of 39K and 41K in molten KNO3 has been measured over the range 354° to 586°C. The mass effect, μ= (Δb/b)/(Δm/m), becomes larger when the temperature is increased, following the relation —,u =0.0385+0.000124 (t-337) where t is the temperature in °C. Due to thermal decomposition, the nitrate is partly converted to nitrite, but it is proved by performing experiments with different initial concentrations of nitrite, that the isotope effect for potassium is not influenced noticeably by the concentration of the anions. The experiment is designed to give an enrichment of the heavy isotope 41K in a small anode compartment and in the upper part of the separation tube. However, it was possible to establish that a slight, but significant, enrichment of the light isotope 39K was obtained in the lower part of the separation tube, i. e. just above the opening into the large cathode compartment. A separation factor of 1.003 was estimated for this enrichment effect, which is due to non-ideal conditions of the experiment.