Unsupported silica membranes were prepared by acid-catalyzed hydrolysis and condensation of tetraethylorthosilicate. The possibility of tailoring the pore size and porosity of microporous silica membranes was investigated by varying the amounts of reactants. A wide range of porosities of 0 to 30% could be obtained. Of the varied parameters, the acid concentration in the reaction mixture is of most importance for the variation in porosity. The pore size distribution was calculated from nitrogen adsorption isotherms, by using the Horváth¿Kawazoe method. This resulted in an average pore diameter of 5 Å with a broad tail up to 10 Å.