Poly(methyl methacrylate) (PMMA)-SiO2 Composite membranes were prepared by casting ternary solutions obtained by adding additional amounts of TEOS to PMMA solutions. Methods to obtain an intimate dispersion of the inorganic particles in the PMMA solutions were developed. Various samples of hybrid membranes were prepared by varying some of the preparative parameters such as the PMMA solvent (tetrahydrofuran, and N,N-dimethyl formamide) and the PMMA/TEOS ratio in the ternary solution. Membranes were also cast from a binary solution of PMMA and solvent for comparison. The membranes were characterized by water permeability, scanning electron microscope observations, DSC and TGA analysis and ultrafiltration tests. The effects of the above-mentioned preparative parameters on the structure, thermal behavior, flux and dextran retention properties of the membranes are discussed.