Abstract: The characteristics of Cs⁺ biosorption by Saccharomyces cerevisia was investigated, including the biosorption kinetics, biosorption equilibrium, isotherm as well as the IR spectrum of biomass pre- and post-biosorption. The experimental results show that the process of Cs⁺ biosorption onto the biomass of Saccharomyces cerevisia can be devided into two stages, the first stage is physical sorption and the sorption equilibrium is very quickly reached (within 20 min). The biosorption kinetics can be described by the pseudo second-order equation quite well (R²=0.989), the kinetic parameters k₂ and qe are 3.56×10^－3 g/(mg•min) and 7.18 mg/g, respectively. The equilibrium isotherm data can be fitted with Langmuir and Freundlich models, with the maximum biosorptive capacity of 10.13 mg/g. Both the IR spectra of the biomass pre- and post-biosorption almost are same, and it indicates that the biosorption of Cs⁺ does not change the structure of the biomass, however, some adsorptive peaks shift.