A series of novel composite electrolytes, consisting of doped ceria (DCO) and salts were developed and examined. The salts included chlorides and carbonates. Their oxygen ion and/or proton conductivity at intermediate temperature (IT) range (500-700 degreesC) can be even 2-15 times higher than DCO. Furthermore, they also show higher ionic transference number than DCO. The long-term stability of fuel cells based on these composite electrolytes was also discussed.