Barium cerate (BaCeO3) is a parent material for the family of high-temperature protonic conductors. Nowadays, BaCeO3-based materials were accessible in ceramic form. Recently, pure and doped BaCeO3 were obtained in crystal form, and this gives an ability for more reliable study of microscopic aspects of hydrogen intercalation. This paper is devoted to study the details of proton intercalation process by luminescent and IR absorption spectroscopy. In this study, it was shown for the first time that charge compensation in the process of proton incorporation in undoped barium cerate is provided by mixed valence state of cerium ions. (C) 2003 Elsevier B.V. All rights reserved.