In a wide pH range,the effect of fluoride ions on the rate of polymerization of silicic acid (acidified water glass and sodium silicate) has been studied.It is found that in neutral and slightly alkaline solutions,fluoride ions promote gelation of silicic acid and the promotion action increases throughout with the increase in amount of the fluoride ions,but in acid solutions,although fluoride ions accelerate the rate of polymerization of silicic acid too,the accelerating action increases only to a certain point when the amount of the ions added has reached a definite value,and when this point.is over,the accelerating action decreases as more of the ions is added.Fluoride ion content of the syneresis liquid of the gels formed at different pH regions has been determined.It is found that the syneresis liquid of the gels formed in alkaline solutions contains all the fluoride ions that are present in the original solution,whereas he syneresis liquid of the gels formed in acid region does not contain any fluoride ion.On the basis of the mechanism proposed for the polymerization of silicic acid in a previous paper of this series,an explanation of the above findings is offered.