Apatite-type systems have been attracting significant attention recently due to their high oxide ion conductivity, offering the potential for use as the electrolyte in a solid oxide fuel cell (SOFC). In this article, we report the synthesis and conductivities of a range of novel mixed Si/ Ge-based apatite systems, La8.67BaSi6-yGeyO26 and La9.33Si6-yGeyO26 (0less than or equal toyless than or equal to6). The results suggest that problems due to Ge volatility increase with increasing Ge content, but are partially reduced by Ba doping, which helps to stabilise the hexagonal apatite lattice. All the phases show high oxide ion conductivities, although the activation energy for oxide ion conduction shows a general increase with increasing Ge content. The mixed systems show the highest conductivities (e.g., sigma(800) (c)=0.06 S cm(-1) for La9.33Si2Ge4O26), and so are promising materials as electrolytes for SOFCs if problems due to Ge loss can be overcome. (C) 2004 Elsevier B.V. All rights reserved.