Novel composite materials based on mixtures of samarium-doped ceria (SDC)-carbonate were examined for use as electrolyte materials in low-temperature solid oxide fuel cells (LTSOFCs) operating at 400-600 degrees C. The composite electrolytes showed high ion conductivity at evaluated temperatures. Composition and calcination temperature were found to affect the morphology and conductivity of the composite electrolytes greatly. During fuel cell operation, water was observed at both electrodes, indicating that the new electrolyte materials conduct both oxygen ions and proton simultaneously. According to fuel cell performance, these composite electrolytes are chemically stable, which is an attractive prospect in LTSOFC applications. (c) 2005 The Electrochemical Society. All rights reserved.