Apatite-type phases, e.g. (La/Sr) 10-x Si 6 O 26+y , are attracting significant interest as an important new class of oxide ion conductor. The highest conductivities have been observed for the germanium based analogues, (La/Sr) 10-x Ge 6 O 26+y , although these systems are complicated by germanium volatility problems. In this paper results are presented on mixed silicon/germanium systems, La 9.33 Si 6-x Ge x O 26 and La 9 BaSi 6-x Ge x O 26.5 (0 ≤ x ≤6). We discuss how factors such as conductivity and germanium volatility vary with silicon to germanium ratio. In the former system, the highest conductivity was observed for x = 4, i.e. σ 800°C (La 9.33 Si 2 Ge 4 O 26 ) = 0.06 S cm -1 , while for the latter, the highest conductivity was observed for x = 0, i.e. σ 800°C (La 9 BaSi 6 O 26.5 ) = 0.05 S cm -1 . Data on related titanium doped systems are also reported, and this substitution is shown to have a detrimental effect on the conductivity.