This relative-pressure sensors has virtually no zero offset at relative humidities up to near the saturation limit. The sensor has a diaphragm (11) having a surface on which a electrode (13) is deposited and a substrate (12) having a bore (23) for guiding reference air from af first surface to an opposite surface. The first surface is polished and provided with at least an addional electrode (15). The substrate and the diaphragm are soldered or brazed together along the periphery by means of a spacer (20) to form a chamber which is covered from inside with a thin layer (24) of hydrophobic material that is introduced through the bore after the soldering or brazing.