A model for the dissolution of oxides has been developed recently. It has been tested with silica. The model describes the overall rate constant of dissolution as composed of several reversible elementary reactions. By this way the overall rate constant of dissolution can be expressed in dependence on pH and ionic strength. It is also possible to derive an expression for the rate constant of polycondensation in dependence on pH and ionic strength. The exact evaluation of these expressions is difficult and only possible by numerical methods. Approximate analytical expressions are therefore deduced, which are valid for special pH regions. The pH dependence of the overall rate constant of polycondensation predicted by the model is confirmed by both synthesis of silica sols from alkali silicate solutions and synthesis from tetraethylorthosilicate.