ZnO-Al2O3 nanocomposite thin films were prepared by sol-gel technique. The room temperature synthesis was mainly based on the successful peptization of bochinite (AlO(OH)) and Al(OH)(3) compounds, so as to use it as matrix to confine ZnO nanoparticles. The relative molar concentrations of xZnO to (1 - x) Al2O3 were varied as x = 0.11, 0.2 and 0.5. The optical absorption spectra of the thin films showed intense UV absorption peaks with long tails of variable absorption in the visible region of the spectra. The ZnO-Al2O3 nanocomposites thin films were doped with MgO by varying its molar concentrations as y = 0.05, 0.75, 0.1,0.125, 0.15 and 0.2 with respect to the ZnO present in the composite. The MgO doped thin films showed suppression of the intense absorption peaks that was previously attained for undoped samples. The disappearance of the absorption peaks was analyzed in terms of the crystalline features and lattice defects in the nanocomposite system. The bulk absorption edge, which is reportedly found at 3.37 eV. was shifted to 5.44 eV (for y = 0.05), 5.63 eV (for y = 0.075) and maximum to 5.77 eV (for y = 0.1). In contrast, beyond the concentration, y = 0.1 the absorption edges were moved to 5.67 eV (for y = 0.125), 5.61 eV (for y = 0. 15) and to 5.49 eV (for 3, = 0.2). This trend was explained in terms of the Burstein-Moss shift of the absorption edges. (c) 2007 Elsevier B.V. All rights reserved.