Hybrid cellulose acetate (CA) silica (SiO2) (CA/SiO2) membranes were synthesized by promoting the in situ condensation between silanols from the SiO2 precursor and the COH or acetate groups from the CA polymer. For all the CA/SiO2 membranes, the ATR-FTIR peak assigned to (SiOC) proves the hybrid condensation reaction and confirms the synthesis of monophasic hybrid membranes. ATR-FTIR shows the presence of uncondensed highly reactive SiOH species, in membranes with silica contents higher than 20 mol%. Together with RMN studies, results show molecular water strongly hydrogen-bonded with SiOH groups, yielding a drastic decrease in the membrane hydraulic permeability, from 57 to 10 kg/h/m2/bar. The incorporation of 5 and 10 mol% of silica increased the hydraulic permeability from 32 to 82 kg/h/m2/bar when compared to the CA membrane.