The neutron diffraction study was carried out to determine the location of protons in the crystal of a proton conducting mixed perovskite oxide Ba3Ca1.18Nb1.82O8.73. The sample was synthesized by a method of solid state reaction, and then the sintered sample was heated for the two experiments; for dried and deuterated samples. The neutron diffraction experiments were performed using HRPD at JAEA for the dried and deuterated samples at 300 K. The obtained data were analyzed by Rietveld method and maximum entropy method. It was concluded that deuterium existed at 96j site, which was shifted to the Ca/Nb mixed site. (c) 2008 Elsevier B.V. All rights reserved.