Separation membranes with high performance can potentially be made by incorporating zeolites (or other nanoporous molecular sieves) in polymeric materials. However, the fabrication of technologically viable membranes has been hampered by poor adhesion between the inorganic crystals and the polymer and by inadequate dispersion of the inorganic particles. We report a facile, high-yield, and inexpensive solvothermal deposition process to prepare roughened inorganic Mg(OH)(2) nanostructures on zeolite (MFI) crystal surfaces in a controlled manner. The functionalized zeolite crystals result in high-quality "mixed matrix" membranes, wherein the zeolite crystals are well-adhered to the polymeric matrix. Substantially enhanced CO(2) and CH(4) gas permeation characteristics were observed in mixed matrix membranes containing up to 35 wt % of solvothermally modified MFI crystals. Gas permeation measurements on membranes containing nonporous uncalcined MFI revealed that the performance enhancements are indeed due to significantly enhanced MFI-polymer adhesion and distribution of MFI crystals.