Two series of experiments have been conducted to obtain the gas-liquid equilibrium partition coefficient Kd and the nonequilibrium partition coefficient K’d of volatile fission products such as cesium, iodine, and tellurium between liquid sodium and the gas phase. In the equilibrium experiment, a sodium pool mixed with a fission product simulant was heated by an electric furnace, and the solvent of the vapors and aerosols trapped by filters was quantitatively analyzed. The results are as follows: 1. Cesium shows the largest Kd (20 to 100). 2. The Kd value of iodine scatters as widely as 0.02 to 0.5 at 450°C and 0.3 to 0.8 at 650°C. 3. The Kd values of cesium and iodine agree well with the theoretical ones reported by Castleman and Tang. 4. If sodium telluride, which is harder to vaporize than pure tellurium, is assumed, the measured Kd value of tellurium agrees with the theoretical.The nonequilibrium experiment in which the temperature dropped relatively sharply in the cover-gas region shows that K’d was not larger than Kd.