A new theory of glass membrane electrode response based on a solid state model is illustrated by application to lithium and sodium responses of five commercially available low-error pH glass electrodes. Mobility, ion exchange, and overall selectivity parameters have been calculated at several interference activities and temperatures from 6 to 55 °C. Beckman sodium selective electrode responses were measured for hydrogen, lithium, and potassium ions and characteristic parameters calculated. For the latter electrodes, conditions giving slow response, non-Nernstian slopes, and long-term drift were explored. The pH glass results suggest that the cation mobilities exceed those of protons while the reverse is found for the sodium-selective glass. The overriding effect of ion exchange selectivity follows from the theoreti-