The effect of variation of temperature and of pH on the condensation polymerisation of silicic acid, prepared by ion-exchange method, have been studied separately. The Arrhenious equation for determination of the activation energy has been employed and the values of E obtained are 10,000 calories, when water is the dispersion medium, and 9,000 calories, when mixtures of water and ethanol and water and glycol are the media. The maximum time of gelation of a particular silicic acid sol is at a pH equal to that of a pure acid (where no extra amount of strong acid or base has been added). This pH lies between 2 and 3, depending on the concentration of SiO2 in the sol. On either side of this pH, the time required for gelation is reduced due to catalytic action of H+ or OH— ions.