A measurement device is provided for determining the oxygen activity in molten metal or slag with a measurement head, which is arranged at one end of a carrier tube and on which an electrochemical measurement cell is arranged. The electrochemical measurement cell has a solid electrolyte tube, which is closed at one end and which contains a reference material and an electrode at its closed end. The electrode projects from the opposite end of the solid electrolyte tube. The outer surface of the solid electrolyte tube has a coating of a mixture of calcium zirconate with a fluoride. The measurement device can be used for calculating the sulfur, silicon and/or carbon content of the molten metal or slag from reaction of the sulfur with the calcium zirconate, which releases oxygen, and measurement of the resulting change in oxygen activity of the melt or slag at the outer surface of the solid electrolyte tube.