[en] The diffusion coefficient of cesium iodide vapor in rare gases was determined by a modified Stefan's method. The rare gas in a diffusion column was saturated with vapor of the cesium iodide, crystals of which were heated to melt at the bottom of the column. By opening a valve united at a top of the column, the vapor diffusing through the column was transferred with the carrier rare gas to an ionization sensor. The concentration of cesium iodide in the carrier gas was continuously monitored with the sensor. The diffusion coefficient was determined by analyzing the transient response of the concentration. Increasing tendency with temperature is observed in the coefficients obtained in argon, krypton and xenon at temperatures between 631 and 691degC and no significant difference among the coefficients in argon, krypton and xenon. (author)