The interaction between Ce-0.90Gd0.10O1.95 (CGO) and Y0.16Zr0.84O1.92 (YSZ) and the electrical conductivity and thermal expansion for solid solutions of CGO and YSZ have been studied. Thin films of CGO interacted with YSZ substrates around 1000 degreesC, with diffusion of Zr into CGO. The reaction between CGO and YSZ powder started around 1200 degreesC. The diffusion of Cc into YSZ was the main reaction. The solid solutions of CGO(x)-YSZ(1-x) were single phase and possessed a lower conductivity than that of either CGO or YSZ. The electrical conductivity for the solid solutions increased with decreasing oxygen activity, whereas the pO(2), independent ionic conductivity region was not observed. The dimensional expansion due to reduction (chemical expansion) was larger for the solid solutions compared to that for of CGO and YSZ. (C) 2004 Elsevier B.V All rights reserved.