A method and a device for detecting a gaseous anhydride and measuring its concentration in an oxygen-bearing gas. The method comprises the following steps: (a) forming an electrolytic junction by contact between a first solid-electrolyte element containing oxyanions of the anhydride to be detected and a second, 0.sup.-- ion-conducting solid-electrolyte element such as stabilized zirconia; (b) bringing this electrolytic junction into contact with the gas containing the gaseous anhydride to be detected so as to form a triple junction; (c) creating at this triple junction a difference of potential measurable by means of two reference electrodes in contact with the first and second electrolyte elements respectively, by fixation of a constant potential in the vicinity of each of these reference electrodes, which themselves are spatially removed from the triple junction; (d) heating the triple junction to such a temperature that a logarithmic variation in the concentration of the anhydride to be detected produces a proportional, substantially linear variation in the difference of potential at the triple junction, this temperature being lower than the melting temperatures of the first and second electrolyte elements; and (e) measuring this difference of potential by means of a potentiometer connected to the reference electrodes so as to obtain a measurement of the concentration of the anhydride to be detected. This method and device may advantageously be used for detecting sulfur carbon anhydrides.