Montmorillonite adsorption experiments were conducted in the fluoride solution. The results show that an excellent negative correlation exists between the adsorption ability of montmorillonite and pH values of the solution. Montmorillonite adsorbs large quantities of fluorine ions and the defluoridation effect is remarkable at low pH value. With the rising of the pH value, the amount of fluorine adsorbed by montmorillonite decreases rapidly and, what is worse, montmorillonite seems almost not to adsorb fluorine ions at all in basic-neutral solution. The amount of montmorillonite and the concentration of fluoride solution also affect the amount of fluorine adsorbed on montmorillonite. This is believed to be the result of a positive surface charge build-up on the montmorillonite at low pH that causes affinity of the negatively charged fluoride ions. In addition, when the concentration of the fluoride solution increases, the electric double layer becomes thin and theζpotential falls, resulting in flocculation of montmorillonite together with fluoride ions. In addition, the ionic exchange between the fluoride ions and OH-is an important factor responsible for the adsorption of fluorine onto montmorillonite in acid solution.