electrodes of an interdigitated electrode pattern formed by gold electrodes 5 and 7 as shown in fig1 also shown are terminals 9 and 11 which couple the electrodes 5 and 7 to an external device the electrode pattern was approximately 4 mm on a side with ten gold fingers 0 2 mm wide and spaced 0 1 mm apart the sensor was formed in standard manner the nafion film had a thickness of 2 micrometers the humidity sensor 1 of fig1 was then dipped into a 0 5 molar solution of ammonium chloride for 15 minutes the film was then rinsed in deionized water air dried cured at 135 degrees c for 1 hour and conditioned by cycling the relative humidity between 30 and 90 percent at 24 and 49 degrees c the sensor was then tested under exposure to dilute alcohol vapor vapor above a solution of 5 ethanol 5 methanol in water for 101 days the stability under this exposure was comparable to that of the best commercially available sensors the results of exposure of the nh4 exchanged sensor to dilute alcohol vapor is displayed in fig2 even after more than 3 months the response characteristic remained within a narrow band the resistance in kilohms at 1 khz 24 degrees c and 33 relative humidity was measured across the humidity sensor of example 1 prior to ion exchange with nh4 and found to be 10 000 kohms after ion exchange with the nh4 the resistance was 5 000 kohms example 1 was repeated except that the humidity sensor was dipped into a 0.