The structural and optical properties of Zn1-xMgxO films deposited by ultrasonic spray pyrolysis were studied. All the Zn1-xMgxO thin films maintained the ZnO wurtzite structure and had no impurity phase, even for the Mg content up to x = 0.27. The optical properties were characterized by transmittance, absorption spectroscopy and photoluminescence measurements. For all the films, the average transmission in the visible wavelength region (lambda =400-800 nm) was over 85%, and the absorption edge shifted to a shorter wavelength as the Mg content increased. The optical energy band gap of Zn1-xMgxO thin films, measured from transmittance spectra, could be controlled between 3.29 and 3.58 eV by adjusting Mg contents. The photoluminescence emission peaks of Zn0.94Mg0.06O and Zn0.73Mg0.27O thin films were located at 369 and 349 nm, showing an evident blue-shift with Mg content increase. The room temperature absorption and photoluminescence properties of the films were also discussed. (c) 2005 Elsevier B.V. All rights reserved.