The gelation of solutions containing mixtures of tetraethoxysilane (TEOS) and polydimethyl siloxane (PDMS) with isopropanol, H2O, and HCl as the catalyst was studied at various temperatures in closed systems. Large variations in the gelation rates were observed with varying temperatures, and different acid and water contents. The gelation rates were found to increase with increasing temperature as well as with HCl content. However, decreasing the water concentration (the molar ratio H2O:TEOS < 3) had a more significant effect in decreasing the gelation rate. Apparent activation energies were obtained from Arrhenius plots of gelation time against temperature and they were shown to change with acid content. The microstructural evolution in the final, dried gels appeared to be related to the degree of network linkage in the present system.