A commercially available pi-I-responsive glass specimen with the molar composition of 25Li(2)O-5BaO-3La(2)O(3)-7Ti0(2)-60SiO(2) was subjected to heat treatment in a vacuum furnace and the pH response of the heat-treated glass specimen was compared with that of the as-received glass. It was found that the heat-treated glass specimen in which the content of "mobile" hydrogen ions was decreased showed a much lower Nernstian slope than the ideal value (59 mV). From these findings, we conclude that the generation of the pH selectivity originates from a hydrogen-ion concentration cell between a solution of a given pH and a glass electrode material and that an essential factor for a glass specimen to exhibit the pH response is that the glass should contain "mobile" hydrogen ions.