Monosilicic acid of sufficient concen kration readily sets into a gel. When the logarithm of gelation time(lgt) is plotted against the initial pH of the acid solution in a wide enough rango, a N-shaped curve would result. According to the theory of polymerization of silicio acid proposed by us previously, the pH values at the maximum and minimum points of the curve beax inherent rely Eions with the dissociation constants of the various species of the monosilicic acid which are involved in the polynaerzation process.