Following recent reports of an enhancement in the conductivity of apatite-type germanate systems at intermediate temperatures in wet atmospheres, the ability of a range of apatite-type silicates/germanates to incorporate water has been assessed through a combination of X-ray diffraction studies and thermogravimetric analysis. The results indicate that the germanate apatites allow higher water incorporation, leading to significant changes in cell parameters. In particular, the main unit cell expansion on water incorporation is along a, b, which would be consistent with accommodation of water within the GeO4/SiO4 framework, rather than down the central channels (for which a large expansion along c would be expected). The consideration of the defect equations for water incorporation shows that in addition to the introduction of protons, water incorporation results in additional oxide ion interstitial defects, and thus this fact should be taken into account when explaining any changes in conductivity in wet atmospheres. (C) 2008 Elsevier B.V. All rights reserved.