High-temperature, gas-cooled, graphite-moderated reactors of the type represented by the HTGR feature a continuous removal of volatile fission products by, and their subsequent trapping from, a helium purge stream. Cesium is a volatile fission product of considerable interest; therefore, an investigation of the specific sorption (gm Cs /gm C) of activated charcoal as a function of temperature and pressure was undertaken. The experimental approach was to use Cs137-tagged metal of known specific activity whereby the amount of cesium sorbed on charcoal could be determined in situ by a calibrated gamma-ray spectrometer system. Cesium adsorption on activated charcoal was found to follow the Freundlich adsorption equation. Isosteric heats of adsorption are given as functions of specific adsorption. The free energy of adsorption was found to be a linear function of the specific adsorption only, thus leading to a method of determining the adsorption isobars and isotherms from a minimum of data.