During oil or water drilling, the evolution of gaseous hydrogen sulfide (H2S) may occur. A better solution would be to prevent the evolution of the gas by simultaneously measuring the presence of sulfide ions (S2-) in the drilling mud and the pH, sulfide ions being predominantly under gaseous H2S form when pH is smaller than 7. We proposed then to perform in situ measurements, using a potentiometric technique, to simultaneously record the presence of sulfide and the pH of the mud. In case of a rapid decrease of pH, human action could be done to rapidly increase the pH and to prevent H2S evolution. Anew type of all-solid-state reference electrode based on thick films of lithium lanthanum titanium oxide (LLTO) has therefore been developed. This reference electrode is associated with a solid state pH electrode and an ionic selective electrode for the detection of sulfide ions. (C) 2013 Published by Elsevier B.V.