Regularities of dissolution of colloidal SiO2 particles in the formation of concentrated polysilicate solutions from stabilized silica sols are studied. It is shown by nephelometry, IR spectroscopy, and titration analysis that the initial structure of polysilicate solutions prepared from the sols containing compact waterless colloidal SiO2 particles is destroyed with time: particles are dissolved, and soluble silica species are accumulated. Viscosity of solutions increases with time, and gels are formed at a certain stage. In polysilicates with a large specific surface area and high particle activity, most of the silica is transferred into a solution before the gelation; dissolution slowly proceeds also at later stages. Probably, the resultant system does not contain SiO2 particles.