Hybrids based on cellulose acetate (CA) and SiO2 were prepared by hydrolysis of tetraethoxysilane (TEOS). More rigid films were obtained with an inorganic phase incorporation. The thermal stability of the hybrids was similar to pure CA. Composite membranes were prepared by casting of CA/TEOS mixtures onto a poly(vinylidene fluoride) support. The water permeation decreased with the incorporation of the inorganic phase. Hybrid membranes were able to retain solutes with a molar mass of congruent to 9000 g/mol ( congruent to98% retention). Hybrids were submitted to biodegradation tests. The presence of the inorganic phase did not inhibit the growth of Thricoderma harzianum fungi. (C) 2002 Wiley Periodicals, Inc.841221962205