Humidity sensing mechanism of sensors based on electrospun polyaniline (PANI) nanofibers was investigated. Dynamic adsorption and desorption behavior of the PANI nanofibers under different humidity levels were monitored by quartz crystal microbalance measurements and compared with their impedance response. It was found that the nanofiber sensor exhibited fast, reversible and highly sensitive impedance change as a function of water molecules adsorbed. Nyquist plots and Bode plots of the nanofiber humidity sensor under different humidity levels were obtained by electrochemical impedance spectra measurements at room temperature. An electrical equivalent circuit was established to model the impedance response to humidity variation of the nanofiber sensor. Calculated results using the model fitted quite well with the measured spectra. Exploration of the sensing mechanism was attempted by correlating the variation with humidity of three components of the equivalent circuit to the morphology of the electrospun nanofiber sensors. (C) 2012 Elsevier B.V. All rights reserved.