In the present study, the appropriate electrolysis conditions were determined for attaining doped ceria thin films with a high density and adhesion in an aqueous solution containing of Ce3+ and Sm3+ ions. Based on a comparison of the anodic and cathodic polarizations, while the former only induced the deposition of Ce3+, the latter accomplished the simultaneous deposition of Ce and Sm species. Under an applied cathodic bias below the hydrogen evolution potential, the Ce3+ and Sm3+ were reacted with OH-ions generated by the reduction of water molecules, and then were deposited on the electrode as a hydroxide. The hydroxide was subsequently oxidized and dehydrated to form the ceria-based thin layer. The morphologies of the as-deposited films were significantly altered on the basis of the applied potential. Moreover, the addition of acetic acid to the electrolysis bath caused the production of a transparent, dense, and adherent film. The XRD pattern and Raman spectrum of the thin film revealed that the film was crystallized as the fluorite structure without any heat treatment, and Sm3+ is substituted at the Ce4+ site. Moreover, the Sm content in the film could be easily controlled by the metal concentration in the solution. (C) 2009 Elsevier Ltd. All rights reserved.