The present paper deals mainly with the released mechanism of hydroxo group in the reaction of soils and clay minerals with neutral NaF solution. On the basis of the study, the method used for determining the hydroxo group and aquo group on the surface of sail colloids was modified. The experimental results showed that the release of hydroxo group from ligand exchange by F^- ions in the reaction between the samples and NaF solution was obvious, and most of hydroxo groups were released due to the formation of cryolite on the surface of soil colloids. The amount of hydroxo group released from complex solution was little and difficult to be distinguished from that released due to the solution of cryolite. Results of experiment showed that due to rapid reaction of samples treated with NaF solution for 2 hours, it could be minimized the influence of cryolite formation on the amount of hydroxo group released from ligand exchange to minimum. Therefore a suggestion of 2 hours for extraction in the procedure of determining the hydroxo group may be acceptable.Within the time for extraction mentioned above, from the difference between the amount of F^- ions absorbed by the samples and the amount of hydroxo group released from the samples, the amount of aquo group might be calculated. The influence of lig- and exchange between F^- and phosphate or silicate ions of the surface of soil colloids on the ambunt of aquo group may be negligible.