Bismuthized platinum electrodes were investigated as pH indicating electrodes in strong KOH solutions (pH 14–16). The E-pH curve of freshly prepared bismuth electrodes is linear, but the E-pH slope altered with time. It is shown that the slope is constant if the bismuth electrode has been aged via immersion in a concentrated KOH solution. The value of the E-pH slope depends on the KOH concentration of the ageing electrolyte; a higher KOH concentration resulting in a less negative value of the E-pH slope. The pH of strong alkaline solutions can be monitored with these electrodes for several hours. The bismuth electrode surface morphology and composition were studied with ESCA and SEM techniques. It is suggested that at freshly prepared bismuth electrodes the formation of the bismuth trioxide or trihydroxide is the potential determining process. At aged electrodes, the potential determining process may involve various bismuth oxide species, but a reaction occurring via the solution cannot be excluded.