Tip potential (TP) of glass microelectrodes filled with 3 M KCl increased remarkably with the increase in the storage period in 3 M KCl solution at 37 degrees C, while the electrode resistances decreased gradually. 2. The electrical conductivity through the thin glass wall near the tip was found to increase in parallel with the TP increase. 3. The e.m.f. across the thin glass wall in the tip region was directly measured. This seems to contribute to the TP generation of the microelectrode when the conductivity of the glass wall is significantly high in the tip region. 4. Effects of the acid treatment of glass employed and the acidification of fillant electrolyte solution suggested that fixed negative charges on the glass wall play a fundamental role in the TP formation. 5. Based on these experimental results, it was concluded that not only the diffusion potential through the tip pore but also the interfacial potential through the thin glass wall near the tip contributes to the TP generation, and the contribution of the latter increases with a long exposure period of the electrodes to electrolyte solution. 6. In this connection, technical problems related to reduction of the tip potential were also discussed.
