A humidity sensor based on a thin-film bulk acoustic wave resonators (FBARs) operating at sub gigahertz frequencies has been developed in this work. The FBAR was microfabricated, electrically characterized, and then coated with the hygroscopic polymer poly[vinylpyrrolidone] (PVP). The FBAR was electrically connected to 50 Omega transmission lines and sealed within a temperature stabilized environmental isolation chamber. Changes in the moisture content of a nitrogen stream were measured with FBAR and verified with external hygroscopes. Response time, sensitivity, and hysteresis of the prototype sensor were examined. In some experiments, the thickness of the PVP polymer film exceeded the total thickness of the FBAR, yet the device still continued to resonate with sufficient quality Q factor to allow for continuous tracking of frequency changes in response to humidity change. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3496003] All rights reserved.