Cerium compounds have been identified as leading candidates to replace hexavalent chrome as conversion coatings on aluminum alloys to improve corrosion resistance. Cerium also shows promise for use as an inhibiting pigment in paint systems. The cerium conversion coatings can be deposited using either spontaneous or nonspontaneous electrolytic processes. In both cases the protective cerium oxide film forms by a precipitation mechanism that is very dependent on electrochemical potential and pH. The oxidation state and phase of the condensed cerium has been shown to be an important aspect of the corrosion protection properties provided by the film. Because of the strong influence of the solution chemistry and operating parameters on film performance, a basic knowledge of the system stability is essential. Toward this end, a revised E-pH diagram was developed for the Ce-H2O system. The Ce-H2O-HClO4 system was chosen as an example system in which hydroxy ions are the only significant complexing species for the Ce ions. A stability diagram was constructed using more recent thermodynamic data on cerium species not available for the original diagram published in Pourbaix's atlas. Significant differences were noted between the previously published Ce-H2O diagram and the one presented here. Precipitation tests were carried out to verify the trends indicated in the new diagram. The importance of the updated E-pH diagram in understanding the formation processes of cerium conversion coatings are discussed.