Abstract The application of solid-sulphide electrolyte sulphur sensing cells to measure directly dissolved sulphur in molten copper has been investigated using an immersion probe with the Cas-2wt% ZrS 2 solid solution electrolyte and the Mo+Mo 2 S 3 two-phase mixture reference electrode. The emf measurements were carried out at 1473 K for the liquid copper with sulphur concentrations from 0.43 to 2.20 at %. Steady potentials were attained within ≈10 min after dipping the probe. The equilibrium partial pressures of sulphur in molten copper calculated from the observed emfs were in agreement with those from the H 2 -H 2 S gas equilibrium investigations.