Antimony electrodes are widely used for gastro-oesophageal pH monitoring. They are also sensitive to oxygen, however, especially at low PO2 levels, which are known to shift recorded values in the alkaline direction. This study, which compares antimony and glass electrodes for oesophageal pH monitoring in six adults, shows that values recorded by antimony electrodes are 2.1 +/- 0.8 pH units (mean +/- SD) higher than by glass electrodes (p < 0.001; n = 7642). A further 52 patients with suspected gastro-oesophageal reflux were investigated by 24-h pH monitoring by means of antimony electrodes. In these patients the oesophageal pH was higher than 8.0 for 7% of the time (range, 0-60%). The alkaline periods recorded with antimony electrodes were all protracted in time, smoothly increasing from a neutral pH, and did not correspond to a sudden increase in pH, which would be expected if alkaline reflux had occurred. It is concluded that high pH values obtained by antimony electrodes are due to the oxygen sensitivity of the electrodes. The diagnosis of alkaline reflux seems to be valid only when pH monitoring is performed with glass electrodes or when values obtained with antimony electrodes are adjusted for the influence of the oxygen tension in the oesophagus.