【Objective】The influence of free iron oxide on the adsorption and desorption of Se(Ⅳ) in a lateritic red soil and red soil in the selenium-rich area of Guangxi was studied. 【Method】The adsorption and desorption characteristics of Se(Ⅳ) in the soil before and after removing free iron oxide were compared by isothermal adsorption and desorption experiments. Zeta potential, scanning electron microscopy and energy spectrum analysis and Fourier transform infrared spectroscopy was used to explore the influencing mechanism.【Result】The results showed that the adsorption process for Se(Ⅳ) fitted the Langmuir and Freundlich models, with correlation coefficients ranging between 0.920~0.995. After removing free iron oxide of red soil and lateritic red soil, the zeta potential became more negative and changed from -24.42 and -18.06 mV to -33.06 and-26.43 mV. Also, the specific surface area was decreased. This observation correlated with the lower adsorption capacities of the soils after the removal of free iron oxide. Hence, the order of maximum adsorption capacity was: lateritic red soil (1 399 mg·kg^-1) > red soil (1 336 mg·kg^-1) > DCB-treated lateritic red soil (444 mg·kg^-1) > DCB-treated red soil (352 mg·kg^-1). The desorption rates of the tested soils were between 2% and 7%, while that of DCB-treated soils were higher than that of the original soils. The FTIR peak fitting analysis showed that the soils reacted with selenium mainly through the oxygen-containing groups such as -OH, Fe-O and C=O. After the removal of iron oxide, the effect of Fe-O in the adsorption was weakened or disappeared.【Conclusion】Free iron oxide can significantly increase the adsorption capacity and strength of soil for Se(Ⅳ) and reduce the release of Se(Ⅳ) by its physical and chemical properties and surface groups.