Abstract: An electrode device for the determination of the partial pressure of CO.sub.2, Pco.sub.2, in particular an electrode device for transcutaneous determination of Pco.sub.2.The interior electrolyte of the electrode device is adapted to the exterior electrolyte in such a manner that the potential difference measured over the exterior reference electrode and the interior reference electrode is substantially temperature-independent with a temperature dependence for the CO.sub.2 partial pressure determination in the range of -1%/.degree.C.-+1%.degree.C.The interior electrolyte preferably contains a pH-buffer system based on phenyl phosphonic acid.