Cd doped ZnO (Cd:ZnO) thin films on the glass substrate prepared by chemical spray pyrolysis technique have been characterized for their optical and electrical properties. The X-ray diffraction and atomic force microscopy results indicate that the crystalline quality degrade due to higher Cd doping in ZnO. The activation energy was found to be decreased when Cd concentration increased. The absorption edge of Cd: ZnO film was found to be red shifted. The direct modulation of band gap caused by Zn/Cd substitution is responsible for the red shift effect in absorption edge of ZnO. The low temperature conduction has been explained by variable range hoping mechanism, which fits very well in the temperature range from 108 K to 301 K. The interaction between Cd and defects in ZnCdO alloy to understand the important roles of Cd in the formation of native defects has also been tentatively discussed. (C) 2012 Elsevier B.V. All rights reserved.