Y3+-doped and Mg2+-doped zirconia thick film humidity sensors were investigated. The humidity sensors exhibited good sensing characteristics. The sensor got a high sensitivity with the impendence changed five orders of magnitude from 10(8) to 10(3) Omega in the relative humidity (RH) range of 11-98% at 20 degrees C. The response time is about 30s for Y3+ doped sensor, and 5 s for Mg2+ doped one, and recovery time is 5 s for both sensors. Small humidity hysteresis is about 3% RH and 4% RH for Y3+ and Mg2+ doped ZrO2 sensors, respectively. Moreover, good repeatability, linearity and temperature properties of the both sensors were also exhibited. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and defect structure were used to analyze the influence of dopant on humidity sensitive properties. And the mechanism of humidity sensing properties was also discussed. (C) 2010 Elsevier B.V. All rights reserved.