Abstract: In the rotary drilling of oil wells a drilling mud is used both to transport the cuttings up to the surface and to impose an hydrostatic pressure on the walls of the borehole. For these functions the mud must for example have an acceptable density and viscosity. It is therefore important to monitor the characteristics of the mud, and to keep them within certain limits. The technique proposed involves the use of an ion selective/reference electrode pair to measure in the mud the potential of the selected ion (as a potential difference), and thus allow a determination of that ion's concentration. Thus, it is a method in which, using a selective/reference electrode pair where the reference electrode is of the type having a liquid junction via an aperture within the reference electrode containment vessel, there is determined the potential difference generated by, and thus the concentration of, the ion in the mud.