When a pH electrode is applied intragastrically for long-term pH metry, problems may occur which are related to the specific conditions of gastric juice and to the possible difference between the temperature of buffer solutions and the body temperature. The fact that pH electrodes measure the activity of the hydrogen ion instead of its concentration may, in concentrated solutions (i.e. pH < 2), lead to an error that is even compounded if other ions are present in high concentrations. This error normally does not influence comparisons between measurements obtained under the same conditions. It is, however, important to check which correction procedures are provided for the particular devices being used when comparing results obtained by different research groups. Calibration of the electrode at a temperature differing from body temperature may result in an important error. Complete correction for this error may be achieved for glass electrodes by a modified version of the Nernst equation; for monocrystalline antimony electrodes an empirically derived formula is available allowing for partial correction. In general, all types of electrodes should be calibrated using two different buffer solutions (one with a pH value between 6 and 8 and another with a pH of 2 or less) and the double-point interpolation method.