This paper describes a neutron diffraction study that has located hydrogen atoms in a proton-conducting perovskite oxide more definitely than ever. The neutron diffraction data were collected on BaSn0.5In0.5O2.75 with and without dissolved D2O at 10 K. Data analysis by the Rietveld method and the maximum entropy method revealed that deuterium atoms were located at or very close to the 12h site of the cubic perovskite structure (space group Pm3m) with an O-D distance of 1.0 angstrom. This hydrogen position is similar to those suggested by the previous computational studies for proton-conducting perovskite oxides. (c) 2006 Elsevier B.V All rights reserved.