In this paper we report further doping studies of the apatite-type phases, (La/Ba)(10-x)(Si/Ge)(6)O26+z, of interest as a new class of oxide ion conductors. We show that it is possible to substitute Ti onto the Si/Ge site, which is, to our knowledge, the first report of Ti substitution on the tetrahedral site in the apatite structure. The conductivities were shown to be highest for samples containing either cation vacancies or oxygen excess, consistent with previous studies of apatite-type oxide ion conductors. However, the Ti doping was shown to generally decrease the conductivity in comparison to equivalent samples with only Si/Ge in the tetrahedral sites, with the greatest decrease for Si containing samples. Further work is required to account for this decrease in conductivity, but one possible explanation could be trapping of oxide ions due to Ti increasing its coordination sphere. (c) 2005 Elsevier B.V. All rights reserved.