A novel and efficient method for molecular engineering of the pore size and porosity of microporous sol-gel silica membranes is demonstrated in this communication. By adding a suitable organic template (e.g. tetraethyl- or tetrapropylammonium bromide) in polymeric silica sols, otherwise known to result in microporous membranes with pores in the range 3-4 Angstrom, we can 'shift' the pore size to 5-6 Angstrom, as judged by single-component gas and vapor permeation results with probe molecules of increasing kinetic diameter (d(k)). The templated membranes exhibit permeances as high as 10(-7) to 10(-6) mol m(-2) s(-1) Pa-1 for molecules with d(k) < 4.0 &ANGS; (e.g. CO2, N-2, CH4), coupled with single-component selectivities of 100-1800 for N-2/SF6, 20-40 for n-butane/iso-butane, and 10-20 for para-xylene/ortho-xylene. The transport properties of the templated membranes are distinctly different from those of the respective silica membranes prepared without templating, and resemble somewhat the transport properties of polycrystalline zeolite MFI membranes prepared by the lengthy, batch hydrothermal synthesis approach, using tetrapropylammonium bromide as a structure directing agent. (C) 2003 Elsevier Science B.V. All rights reserved.