Thin wurtzite (0 0 2) textured ZnO thin films were deposited on glass substrates by radio frequency magnetron sputtering under O-2/Ar ratios R varying from 0.05 to 1.0 at room temperature. The structure of, and defects in, the films were investigated by XRD, SEM and slow positron beam techniques. The XRD spectra showed that ZnO thin films were polycrystalline with hexagonal structure and a good c-axis orientation perpendicular to the substrate. The thickness, grain size and the crystalline quality of the films strongly depended on R; the larger grain size and thicker ZnO films were grown when R was lower. Positron beam Doppler broadening measurements showed that in low R films additional vacancy-type defects (e. g. Zn-related vacancy complexes or clusters) were formed. Photoluminescence spectra found that the film with R = 0.4 had the highest luminescence efficiency, in good agreement with the best c-axis preferential orientation. The transmittance spectra of the films decreased with decreasing R, due to the thickness effect. Correlations between microstructure, defect and optical properties are discussed. (C) 2009 Elsevier B.V. All rights reserved.