Abstract: Fluoride-ion combines with silicic acid only in acid solution, although it catalyses the polymerization of the silicic acid in neutral and alkaline solution. Fluoride-ion concentrations in silicic acid solution of different concentrations and varied F/SiO₂ ratio were measured with fluoride-ion selective electrode. Results of measurements showed that a decrease of [H⁺] was accompanied by an increase of fluoride concentration in solution, i.e. the amount of fluoride-ion combined with silicic acid decreased. At [H⁺]<10^-5M, there was no combination at all. These findings are in agreement with the mechanism which was proposed previously concerning the effect of fluoride-ion on polymerization of silicic acid,In acid solution, when fluoride concentration is far less than that of silicic acid, the chief species of the latter is Si(OH)₄ and that of fluoride is HF and their reaction may be represented as:The for mation constant β₁ of the product formed was found to be. β₁ = 1.7×10³. At high acidity, [Si(OH)₈OH₂]⁺ might be formed and its reac tion with HFis given as:When the ratio F/SiO₂ was higher, the following reaction took place.Si (OH)₄ + 6HF = [SiF₆]^2- + 2H⁺ + 4H₂O. The gross constant, β₆ was found to be: 2×l0¹².