Abstract: There is provided a measuring apparatus for determination of oxygen partial pressure in liquids and gases which comprises a substantially sheathed metallic cathode having a small, unsheathed metallic surface and a metallic anode, an ion impermeable but gas permeable membrane interposed between the material whose oxygen content is to be measured and the unsheathed metallic surface of the cathode, an electrolyte between said cathode and said anode and a barrier layer impermeable to cations of the material forming the anode, interposed between said anode and said unsheathed metallic surface of said cathode. Preferably said metallic anode is a silver anode enveloped with silver chloride being covered with said barrier layer.