The behaviour of a pinhole type sensor for the determination of humidity (1000-28 000 ppm of water vapour, corresponding to about 3.5 to 90 RH% at 25 degrees C) in N-2 and in N-2/O gas mixtures is presented. The operating temperature of the sensor, consisting of a zirconia-based electrochemical cell with a diameter and a thickness of 10 and 5 mm, is 500 degrees C. Imposing a ser of potential (from 0 to 1400 mV) to the electrodes, typical polarographic curves were observed, in the presence of two electroactive species two limiting curves were obtained. Water vapour concentration can be obtained with an error not exceeding +/-4%.