A method of measuring concentration profiles in glass surface layers by photon emission during ion sputtering, that offers a depth resolution of 30 to 50 Å, is applied to leached surface layers of pH-electrode membranes. Two steady state layer regions different with respect to ion diffusion are found at low pH. Within the outer layer the interdiffusion coefficient is constant. Within the transition layer interdiffusion coefficients are observed with a minimum that is two orders of magnitude lower than the lithium diffusion coefficient within the glass and is explained by a low diffusion coefficient of interdiffusing protons within a highly glass-like structure. The explanation is supported by the similar magnitude of the diffusion coefficient obtained here and that of protons which have replaced lithium ions within this glass in an electric field. The corresponding maximum resistivity agrees with measurements by Wikby. Surface layer properties and additional experimental results suggest that the equilibrium determining the pH-membrane potential is the dissociation of SiOH-groups at the immediate layer surface rather than an ion exchange of glass alkali and solution hydrogen ions.