Using platinum electrodes and Nafion as the solid electrolyte, a device for measuring humidity in a chlorine-air atmosphere has been assembled. Deposition of platinum on the polymer is achieved by electroless plating from tetraamine platinum chloride, Pt(NH4)(2)Cl2.H2O, and reduction by sodium borohydride (NaBH4). The sensor was calibrated in terms of impedance as a function of relative humidity, using a Frequency Response Analyser. The frequency range was 10(-2)-10(7) Hz and an amplitude of either 70 or 100 mV was applied. At very low relative humidity, only a bulk process is observed but when the moisture content is increased, an additional process takes place at the electrode/electrolyte interface. This is reflected in the form of a spike at 45degrees to the real axis and represents diffusion of moisture into and out of the platinum/Nafion composite. Maximum conductivity is observed at 433 K. The maximum operating temperature is about 473 K as ascertained by thermal analysis of Nafion. In industrial practice, the sensor can be operated with a simpler frequency controller, which would allow the sensor to be operated at a fixed frequency. The sensor retains its physical and chemical integrity after use in a chlorine-air atmosphere. (C) 2003 Elsevier B.V. All rights reserved.