SnO2 thin films and ZnO–SnO2 composite thin films were grown by reactive magnetron sputtering. Optical emission spectra was used to monitor the sputtered Sn and Zn species in the plasma. The structural and morphological characteristics of the films were studied using X-ray diffraction and scanning electron microscopy. The films were investigated for their humidity sensing characteristics using impedance spectroscopy in the frequency range 100 Hz–5 MHz. The sensing mechanism was evaluated by fitting the impedance spectra with the corresponding equivalent circuit. The composite ZnO–SnO2 based sensors exhibit faster response time (~21 s) compared to that of pure SnO2. The pure SnO2 based humidity sensors possess better sensitivity ((1.10 ± 0.079) kΩ/%RH) whereas the ZnO–SnO2 based sensors exhibit better sensor response (~ 24%). The enhancement in the sensor response is attributed to the dissociative adsorption of water molecules at the Sn and Zn lattice sites resulting in increased protonic conduction. © 2017, Springer Science+Business Media, LLC, part of Springer Nature.