The potential of hybrid organic-inorganic membranes for separating organic molecules from air, based on a solubility-selective mechanism, was evaluated. Alumina membranes with average pore sizes near 5 and 12 nm were surface-derivatized with various alkyl trichlorosilanes, and their permeances to nitrogen, methane, and propane gases were evaluated at feed pressures up to 2 bar. The permeation rates in the modified membranes generally decreased by two to three orders of magnitude compared with the untreated membranes, while the propane/nitrogen selectivity increased significantly. The membrane performance was correlated with the materials and synthesis conditions employed in the modification, which indicated an ability to control the hydrophobic character and free volume of the hybrids. The maximum propane/nitrogen selectivity observed was 32, which compares very favorably to the best results reported for purely polymeric membranes. This study demonstrates that the synthesis of hybrid membranes with tunable free volume and surface chemistry is feasible, and that such membranes may achieve high performance in solubility-based separations.