Since the discovery of H Γ· selective glass electrodes in the late thirties and the advent of glass electrodes selective for other univalent cations in the fifties, the problem of the operative mechanism of glass electrodes has been the subject of a number of opinions and studies 1-12. Current ideas can be roughly classified into two groups. In the first approach, the glass is considered to select from a mixture the cation i with the highest affinity for the surface gel layer sites, and in consequence responds to that ion. This "ion-exchange" theory has been advocated by Eisenman 5, Lengye113, Nikolskii et al. 14-17 and Tendeloo ~8. In this representation, in principle a static equilibrium is attained at the glass-solution interface with the chemical potential difference (~glass --]2~ Β°lutiΒ°n) as the driving force for the generation of the electric potential difference, the electrochemical potential, r/i, being equal throughout. In the second approach, the glass electrode potential is treated as essentially a diffusion potential, i.e. as a typical non-equilibrium phenomenon. In the latter representation, differences in mobility generate a steady-state potential (e.g. Nagasawa and Kobatake xg,