Asymmetric silicon oxide composition profiles along a direction perpendicular to the plane of Nafion(R) sulfonate films were created via in situ sol-gel reactions for one-sided tetraethoxysilane permeation, as verified by EDAX/ESEM. For K+ form membranes, we propose the existence of an IR spectral signature of molecular branches in addition to those characteristic of linear and cyclic fragments in the silicon oxide phase. The molecular structure of the silicon oxide phase is more interconnected than linear in K+ form membranes. For H+ form membranes, there appears to be an increasing degree of molecular linearity within the silicon oxide phase with increasing uptake. IF spectra indicate that molecular connectivity on the permeated side is lower, on the average, than that on the nonpermeated side. The inverse relationship between gas permeability and upstream pressure in steady-state helium gas transmission experiments suggests the dual-mode sorption of gases, which is in harmony with the multiphasic nature of these membranes.