The permeation of hydrogen and helium through vitreous glass and silica membranes is modeled using ab initio density functional calculations. The total energy of the system is obtained as the permeating species approach and pass through H2nSinOn (n=4-8) cyclosiloxane n-membered rings of different size to give activation energies of permeation. Comparison of the calculated activation energies to those reported for vitreous glass (20-40 kJ mol(-1)) indicate the presence of 5- and 6-membered siloxane rings, consistent with the accepted structure of glass as a disordered form of beta-cristobalite. Similar comparison to activation energies measured for silica membranes (10-20 kJ mol(-1)) indicate the presence of 6- and 7-membered rings, and suggest that the structure of the membranes is more open than that of the vitreous glass. (C) 2008 Elsevier B.V. All rights reserved.