To determine fission product-graphite sorption behavior, an experimental and analytical study was conducted to obtain a model which would predict the vapor pressures of mixtures of sorbed fission product metals as a function of temperature and composition.The graphite sorbents studied were SP-1C, a very pure natural flake graphite, and TS-688, a typical needle-coke nuclear-grade graphite. Cesium and rubidium tagged with 137Cs and 86Rb were the sorbates, and the pseudo-isopiestic method was used to determine sorbate concentrations as a function of temperature and vapor pressure. Three models were examined for their capability in predicting binary mixed sorption behavior.Experiments showed the following: (a) The vapor pressure of a species varies inversely with the fraction of total sorbate metal present at a given sorbate concentration and temperature; (b) thermodynamic and FREVAP models were the most successful in the prediction of binary sorption behavior from single component isotherm data. It is concluded that if the departure from ideal adsorption behavior is not known, either model may be used in the calculation of fission product release.