Recent developments on single-phase and composite materials with oxygen ionic conductivity are briefly reviewed. Attention is focused on electrolytes with apatite-like structure, yttria-stabilized zirconia (YSZ) ceramics containing silica and apatite admixtures, and composite mixed conductors where chemical interaction of the components is large or severely constrained. The results show, in particular, a key role of phase interaction and microstructural design on materials performance. The ideas behind the adopted procedures can be extended to other systems and combinations of properties based on thermodynamic equilibria information. (c) 2006 Elsevier B.V. All rights reserved.