Abstract: A method of making a humidity sensor which comprises an alumina substrate bearing a pair of noble metal electrodes. The electrodes are electrically connected to each other by an electrically conductive moisture-sensitive layer present on the substrate. In order to be able to monitor the moisture content of a gaseous atmosphere inside the housing of a sealed electronic device, for example an integrated circuit or an acoustic surface wave device, it is necessary to use a small sensor which can be used in a static atmosphere. According to the method, an electrode system may be formed by sputtering gold onto an alumina substrate. The substrate bearing the electrode system is then baked in air at a temperature in the range of from 100.degree. to 750.degree. C. for from 30 hours to 1/2 hour, and then a coating comprising orthophosphoric acid or phosphorus pentoxide is applied over that area of the substrate which is to bear the moisture-sensitive layer. The assembly is heated at from 50.degree. to 500.degree. C.