A measurement device is provided for determining the oxygen activity in molten metal or slag with a measurement head, which is arranged on one end of a carrier tube and on which an electrochemical measurement cell is arranged. The electrochemical measurement cell has a solid electrolyte tubule, which is closed on one side and which contains on its closed end a reference material and an electrode, which projects from the opposite end of the solid electrolyte tubule. The solid electrolyte tubule has on its outer surface a coating made from a mixture of zirconium silicate with a fluoride. The measurement head may also have a thermo-element, and the device may be used to determine the concentration of silicon and/or carbon in the melt, based on the oxygen activity measurement.