An amperometric pump-gauge zirconia sensor was operated over a range of temperatures in oxygen-nitrogen atmospheres containing various concentrations of either water vapour or carbon dioxide. At pump voltages up to 1 V the pump current showed a rising wave tending towards a limiting value resulting from diffusion of oxygen through the diffusion hole and subsequent electrochemical reduction of this species to oxygen ions; the gauge EMF indicated values up to about 100 mV. At pump voltages between 1 and 2 V the additional species, either water vapour or carbon dioxide, also diffusing into the inner volume of the sensor, was electrochemically reduced to hydrogen or carbon monoxide respectively; the resulting redox couple, either H 2 /H 2 O or CO/CO 2 , raised the gauge voltage by a further 700 mV or so. Provided that sufficient H 2 O or CO 2 was present, the magnitude of the EMF in the second plateau was virtually unaffected by the concentration of oxygen, water vapour or carbon dioxide, or by the temperature of operation of the sensor. This EMF generated additional analytical information, compared with an amperometric sensor without a gauge, and provided increased sensitivity towards the detection of these additional species. Gauge EMF values from the two electrodes in the split gauge arrangement indicated significantly higher oxygen levels within the volume adjacent to the diffusion hole in comparison with levels well inside the sensor.