The humidity-sensing characteristics of ZnO nanostructures are investigated using a quartz crystal microbalance (QCM) measurement. ZnO nanostructures are synthesized via sol-gel route in nanoparticle (ZnO-NP) and nanowire (ZnO-NW) morphologies with diameter about 20-30 nm. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods are used to determine the morphology and crystal structure of ZnO nanostructures. Humidity sensing capabilities are discussed in terms of the different morphologies. The results show that ZnO-NP is more sensitive to humidity changes than ZnO-NW. QCM results are analyzed using Langmuir adsorption model to determine adsorption rates, Gibbs free energy of adsorption (Delta G), and adsorbed mass amount by the synthesized ZnO nanostructures. Negative value of Delta G for humidity adsorption on ZnO nanostructures indicates that the process is spontaneous and adsorption capacity increases with size reduction. Gibbs free energy of the ZnO-NP is found to be more negative, indicating that the ZnO-NP has more favorable adsorption sites compared to the ZnO-NW. Experimental and theoretical results exhibit that humidity-sensing properties of ZnO nanostructures are morphology-dependent. (C) 2012 Elsevier B.V. All rights reserved.