Zinc aluminum oxide films were deposited by reactive dc magnetron sputtering from Zn/Al target containing 30 wt.% Al. Sputtering was carried out on glass and Si(100) substrates held at room temperature. The effect of oxygen flow on structural and optical properties was studied. The properties of the films were strongly dependent on oxygen flow. Amorphous, highly transparent and very smooth films were deposited in the oxidic mode while opaque and high surface roughness films were deposited in the metallic mode. The decrease in refractive index upon increasing oxygen flow is related to both the decrease in density as revealed by X-ray reflectometry and the increase in porosity. The band gaps obtained were in between the band gaps of Al2O3 and ZnO. (c) 2006 Elsevier B.V. All rights reserved.