The thermochemistry of the Er-Cl-H2O system was studied by means of potential-pH, solubility and species repartition diagrams and compared to the Zn-Cl-H2O system in order to predict the codeposition of mixed Zn/Er oxy-hydroxide compounds by electrochemical precipitation. The deposition method consists in increasing the interfacial pH by the reduction of oxygen in solution. The coprecipitation of ZnO and Er(OH)(3) was expected for erbium (III) concentration higher than Zn2+ concentration. To check the predicted behaviour, the Zn2+ bath concentration was fixed at 5 mM and films were deposited at different erbium (III) concentrations. The erbium concentration necessary for the co-deposition was significantly lower than the predicted one. Segregation phenomena were observed. With a pure Er3+ bath, experimental conditions were also found for the continuous growth of an amorphous, porous and fibrous film, whose composition would be Er2O3. (c) 2005 Elsevier B.V. All rights reserved.