Mixed sols were prepared by dissolving polyurethane (a 30 wt% solution in n-propanol, PU) and tetraethylorthosilicate (TEOS) in ethanol at PU:TEOS mass ratios of 1:2, 1:1, 2:1 and 3:1. Each of the sols was coated on a porous a-alumina support tube by the dipping method, and green membranes were heat-treated at 200 degrees C for 1 h in an atmosphere of nitrogen. A PU membrane was also prepared with PU alone. The membranes were 5-6 mu m thick. The polyurethane-silica membranes were swollen in benzene but only slightly in cyclohexane at room temperature. The degree of swelling in benzene decreased with increasing fractions of TEOS in the hybrid sols. The selectivity of benzene to cyclohexane was improved due to the suppression of swelling as a result of hybridization with TEOS. The total permeation flux and benzene/cyclohexane selectivity in the membrane prepared with a sol of PU:TEOS=1:1 were 3 x 10(-5) kg m(-2) s(-1) and 19, respectively.