The family of perovskites Sr3Ca1 + xB2 - xO9 - delta (B = Zr, Ta, Nb...) offer considerable potential as proton-conducting electrolytes that are relatively resistant to carbonation. In this study, we discuss the important structural features of these oxides and demonstrate that the structures of hydrated proton-conducting oxides differ quite significantly from those of unhydrated samples. Subtle changes in unit cell symmetry due to octahedral tilting/distortion, oxygen vacancy filling/creation and cation displacements are all important features accompanying water uptake/loss and play an intrinsic role in the level of proton conduction and mechanism of proton transfer.