Intersubband optical transitions in ZnO-based quantum wells grown by plasma-assisted molecular beam epitaxy are investigated by a photocurrent spectroscopy. ZnO/MgZnO multiple quantum well (MQW) Structures containing different ZnO well thicknesses are prepared. Photocurrent peaks are observed when the polarization of incident light is TM mode, following the intersubband selection rule. On reducing the ZnO well thickness, photocurrent peaks shifted to higher energies. Calculation indicates that the photocurrent peaks are due to intersubband transitions from the first to the third subband in ZnO/MgZnO MQWs. Comparison between experiment and calculation allows LIS to estimate the band offset ratio. (c) 2008 Elsevier B.V. All rights reserved.