Abstract: The montmorillonite is an aluminosilicate mineral which has 2：1 type la yered structure. The cation exchange in interlayer is an important feature. The montmorillonite is treated by means of lithium exchange in this paper. The che mical states of lithium in lithium-exchanged montmorillonite are analyzed by X RD, IR and XPS etc. The results show that the lithium-exchange is different fro m the other cation-exchange in montmorillonite. Because of smaller ionic radius of Li+ ion, Li+ ions can not only exchange with the absorbed cations in mon tmorillonite interlayer, but also replace the cations in the octahedral sites an d enter into the interstitial positions of montmorillonite lattice. There are th ree chemical states of lithium existing in montmorillonite lattice structure. Among them, number of absorbed Li+ ions in interlayer is more than that of Li+ ions in the replacement and interstitial positions.