A series of organic-inorganic hybrid membranes were prepared on microporous alumina substrates through dip-coating with the sol-gel solution of negatively charged polymer precursors PEO-[Si(OEt)(3)](2)SO3H (preparation of which discussed in [C.M. Wu. T.W. Xu. W.H. Yang, Synthesis and characterizations of new negatively charged organic-inorganic hybrid materials: effect of molecular weight of sol-gel precursor, J. Solid State Chem. 177 (4-5) (2004) 1660-16661). Sulfonation degree of PEO-[Si(OEt)(3)](2)SO3H was varied and its effect on the thermal stability, electrical properties and morphology of the membranes were discussed. Results showed that as the sulfonation degree of PEO-[Si(OEt)(3)](2)SO3H increased, T-d of the membranes decreased from 265 to 233 degreesC. while cation exchange capacity increased from 0.4 to 1.0 mmol/.g. Water flux values (0.04-0.51 x 10(-5) L/(m(2) Pa h)) and pore diameter (0.001-0.004 mum) of the membranes generally increased with the sulfonation degree of PEO-[Si(OEt)(3)](2)SO3H. All membranes showed negative streaming potential (SP) even at very low pH values. Thus, negatively charged nanofiltration membranes with relatively high thermal stability and varying electrical properties: were successfully obtained. (C) 2004 Elsevier B.V. All rights reserved.