Recently, apatite-type phases, e.g., La10-x(Si/Ge)(6)O26+y, have been attracting significant interest due to their high oxide ion conduction. Studies of the Ge-based systems have, however, shown them to suffer from problems related to Ge loss at the high sintering temperatures required to obtain dense pellets. In addition, the activation energy for oxide ion conduction is significantly higher in these systems than for the Si-based analogues. In this paper we report a detailed study of the mixed Si/Ge systems, La9.33Si6-xGexO26 to examine how these factors vary with Si/Ge ratio, and so determine the optimum Si/Ge ratio for high oxide ion conductivity. The results show that the activation energy for oxide ion conduction increases with increasing Ge content, with evidence for an intermediate plateau for the range, 2 less than or equal to x less than or equal to 4. Conductivities lower than those previously reported were found for the Ge-based end-member (x = 6). Instead, the highest conductivity at 800 degreesC (0.06 S cm(-1)) was observed for intermediate compositions, x = 4. The results of extended sintering studies suggest that problems due to Ge loss increase with increasing Ge content, with extended heating at high temperatures ultimately leading to the formation of insulating La2GeO5 impurities, resulting in a significant decrease in the conductivity. (C) 2004 Elsevier B.V. All rights reserved.