Detailed X-ray and neutron powder diffraction structural studies are reported for the proton conducting perovskite system "La0.6Ba0.4ScO2.8". Both dry and wet samples were analysed, with the latter prepared by heating in N-2/H2O or N-2/D2O. Although X-ray diffraction suggested that a sample of this nominal composition was single phase, neutron diffraction showed small levels of SC2O3 impurity with the actual composition for the perovskite phase in the dry sample being La0.73Ba0.27ScO2.865. This can be related to Ba volatility at the high temperatures required for the synthesis of this phase, and the result raises important questions regarding many barium containing materials previously reported as single phase by X-ray diffraction. For the hydrated samples, data was collected at 4.2 K and a linked dual refinement for the protonated and deuterated samples was performed in order to provide an accurate location for the HID position, making use of the contrast provided by the fact that the nuclear scattering factor of His negative while that of D is positive. This unique dual refinement proved to be successful in locating the H/D despite the low occupancies present, with a final composition of La(0.73)Ba(0.27)ScO(2.865)0.135(H/D)20 and the H/D associated with the O1 position. The protons and deuteriums are associated with the O1 oxygen site, pointing towards the mixed LaBa position. (C) 2007 Elsevier B.V. All rights reserved.