In this paper a detailed investigation of the effect of oxygen content on the conductivities of Ba doped lanthanum germanate apatites, La8+xBa2-x (GeO4)(6)O2+x/2, is presented. The results show that with no nominal oxygen excess (x=O) the conductivity is low (sigma(800 degrees C) = 2 x 10(-5) Scm(-1)), while the introduction of oxygen vacancies, e.g. La7.8Ba2.2(GeO4)(6)O-1.9, leads to a significant further reduction in conductivity (sigma(800 degrees C) = 7.2 x 10(-7) Scm(-1)). In contrast, a systematic increase in conductivity is observed with increasing oxygen excess content reaching a plateau (sigma(800 degrees C) approximate to 0.02 Scm(-1)) for x >= 1.0. This strong dependence of the conductivity on oxygen excess levels is consistent with the oxide ion conductivity being mediated by oxide ion interstitials, in agreement with previous reports. The effect of cation vacancies on the conductivity has also been investigated, and the results show that the presence of La vacancies also leads to an enhancement in conductivity particularly for samples with low oxygen excess. It is suggested that this is due to the presence of cation vacancies increasing the number of oxygen interstitials by enhancing oxygen Frenkel defect formation. (C) 2007 Elsevier B.V. All rights reserved.