An impedance-type, solid state thick-film humidity sensor based on a NASICON-type proton conductor, HZr2(PO4)(3) has been developed and tested in a wide range of temperature (25-250 degrees C) and relative humidity (2-100%). The film sensor showed enhanced sensitivity compared to the bulk sensor and fast and reproducible response (similar to 10-15 s), even at ambient temperature. Long-term tests of the film humidity sensors show relatively good agreement with the commercial humidity sensor, OMEGA-RH-20C at room temperature, and excellent agreement with commercial Vaisala-sensor at higher temperatures over several weeks. The sensor signal is dependent on the thickness, as well as on the microstructure of the film.