The use of a bilayer composite YSZ-CGO (yttria-stabilized zirconia-gadolinia-doped ceria) electrolyte for intermediate-temperature solid-oxide fuel cells (SOFCs) is still problematic, due to solid-state reaction and interdiffusion phenomena occurring between YSZ and CGO. To overcome this problem, a gradation in the microstructure between the single materials is proposed. To optimize the composition of the interlayer with respect to conductivity, different compositions of the system CGO(x)YSZ(1-x) were prepared and characterized in order to study their transport properties. Conductivity was found to reach a minimum for solid solutions where Zr and Ce are present in equimolar quantities. The latter are, however, the most appropriate candidates to be applied as interlayers between the two single materials. Keeping the Ce/Zr atomic ratio approximately equal to one, different solid solutions were prepared and characterized by varying the concentration of the trivalent cations present, in order to clarify their role in the transport properties of the material. For one of the better conducting systems the cell performance of the composite YSZ-interlayer-CGO film electrolyte is simulated and compared to that of the YSZ-CGO film electrolyte. (C) 2000 Elsevier Science B.V. All rights reserved.