Following the ideas presented in a previous publication [J. Membr. Sci. 187 (2001) 141], hybrid organic/inorganic membranes were prepared by the surface-derivatization of commercially available mesoporous alumina membranes with octadecyltrichlorosilane (OTS). Ideal and mixed gas selectivities for propane/nitrogen and butane/methane systems were obtained at different inlet feed pressures and compositions. The membranes were always selective for the heavier species over the lighter, which is indicative of a solubility-based separation. The mixed gas selectivities were generally greater than or equal to the corresponding ideal values. Interestingly, the dependence of mixed gas selectivity on feed pressure was qualitatively different for the two different gas pairs. Apparent competitive and cooperative effects between the permeating gas species were observed under different conditions. Overall, the permeation characteristics of the hybrids were more consistent with a dense polymeric material rather than a porous one. (C) 2003 Elsevier Science B.V. All rights reserved.