Cadmium oxide thin films with different percentages of aluminum doping have been synthesized via radio frequency magnetron sputtering technique. Thin films were deposited on glass and silicon substrates with different percentages of aluminum at a substrate temperature of 573 K and pressure of 0.1 mbar in Ar + O-2 atmosphere. The deposited films were characterized by studying their structural. electrical and optical properties. The X-ray diffraction pattern revealed good crystallinity with preferred (111) orientation in the films. Aluminum doping in CdO thin films were confirmed by X-ray photoelectron spectroscopic studies and actual doping percentages were also measured from it. The optical band gap was found to decrease first and then increase with increasing percentages of aluminum concentrations. The electrical conductivity was found to increase with increase of aluminum doping concentration up to 5% but for higher doping concentration (> 5%) the conductivity was found to decrease. (C) 2007 Elsevier B.V. All rights reserved.