A direct quantitative measurement of cesium adsorbed into polycrystalline tantalum has been made by a new technique involving radioactive cesium. Cesium vapor was admitted into a known control volume and adsorbed onto two adjacent parallel tantalum disk faces. The amount of adsorbed radioactive cesium was monitored with a sodium iodide crystal &ggr; detector. The surface and cesium reservoir temperatures were independently controlled. Tantalum surface temperatures of 500–1700 °K and cesium arrival rates of 1015–1021atoms/cm2have been investigated. Atom concentration is plotted as a function of temperature for various constant cesium arrival rates. The results follow theoretical predictions up to 0.5–0.75&percent; of a monolayer. At higher coverages, the cesium adsorption increases rapidly to well over one monolayer. The cause of this rapid increase is determined to be the formation of cesium oxide on the surfaces resulting from a calculable diffusion of oxygen through the tantalum test chamber. Although the oxygen content is not measured, the qualitative behavior of the Cs‐O system is shown. Surfaces with coadsorbed cesium and oxygen have a particular usefulness in improving performance of thermionic energy converters.