The solubility of heavy alkali metals in liquid lithium was investigated at temperatures from the melting point of Li to ∼1200 K by the method of gamma-raying of liquid samples by a narrow beam of gamma radiation. Even at high temperatures, the solubility of K, Rb, and Cs in liquid lithium has been found to be very insignificant. The potassium content in the lithium-rich phase is 0.2 at. % at 850 K and 3.4 at. % at 1212 K. The solubility of rubidium in liquid lithium, within the measurement error, is zero at temperatures from 453 K (melting point of Li) to 800 K and reaches only 0.5 at. % at 1172 K. Cesium is practically insoluble in liquid lithium up to a temperature of 1150 K.