An iridium-iridium oxide electrode for in vivo pH measurement of the distal oesophagus is described. It is small and flexible so it is well accepted by patients for long-term pH monitoring. The electrode also offers the possibility of including another electrode and/or a pressure sensor for simultaneous detection of two or more parameters--gastric pH or oesophageal pressure for example. A clinical study was performed on 15 healthy volunteers and the results were checked on a DGC Nova 4/S computer, showing less than 0.005% of tension in mV out of the established range in a period of 24 h pH monitoring. The volunteers' results were compared with the normal values obtained by DeMeester using a glass electrode on a similar, healthy group of American patients and no significant differences were observed. Owing to its small size, reliability, fast response to pH changes, durability and its easy storage, the Ir/IrO2 electrode is ideal for long-term pH monitoring of the upper gastro-intestinal tract.