The catalytic effect of fluoride on the polymerization of sodium silicate and phosphoric acid solutions at pH 7.15 was studied through a simple and inexpensive method: the observation of the meniscus of the resulting sol in a test tube. Reaction was found to be second-order regarding silicate concentration as reported previously. Novel results show that the reaction also follows a pseudo-second-order kinetics with respect to fluoride. A rate law for the catalyzed polymerization of the form r = Kv[Si(OH)4][HF]2 was established. A mechanism for fluoride action on silica polymerization, where silicon increases its coordination number to five, is proposed.