Abstract: The invention relates to a measuring sensor having pump reference for measuring the oxygen content of gas mixtures, particularly exhaust gases of internal combustion engines. The sensor includes a measuring cell (A) and a reference cell (B) each having a pair of electrodes, with one of each respective pairs of electrodes being common to the two cells, and having a fixed electrolyte. Reference cell (B) has defined therein an internal oxygen reference zone which is hermetically sealed from the gas mixture and which is connected to the atmosphere by way of a pressure-equalization line. The measuring cell (A) and the reference cell (B) can be heated by heating apparatus to a temperature at which the fixed electrolyte has an ionic conductivity which is sufficiently high.