The proton compounds Ba2In2(1-x)Ti2xO4+2x(OH)(y) [0 less than or equal to x less than or equal to 1; y less than or equal to 2(1 - x)] were prepared by reacting Ba(2)In(2(1-x))Ti(2x)O(5+x)square(1-x) (0 less than or equal to x less than or equal to 1) phases with watervapor at similar to 200 degreesC. For 0 less than or equal to x less than or equal to 0.20, the filling of oxygen vacancies is almost complete. For larger x values, it decreases significantly down to similar to 30% only for x = 0.7. The crystal structure of the end member Ba2In2O4(OH)(2) (x = 0) was reinvestigated by a combination of techniques including H-1 and D-2 NMR and electron, X-ray and neutron diffraction. The actual cell is eight times larger than that previously published. The structure analysis confirms that the sheet consisting of parallel chains of In(2)O-4 tetrahedra and parallel rows of oxygen vacancies in the parent structure of Ba(2)In(2)O(5)square has been converted into a In(2)O-6 octahedral perovskite-like sheet. It demonstrates that the protons are bonded only to the O atoms around this In(2) site. When x increases, the change in reduced perovskite cell volume, concomitant with the water uptake, remains small for x < 0.25 and negligible for larger x values. The proton conductivity was measured between room temperature and 180 degreesC. The highest conductivity at 180 degreesC, sigma(180) approximate to 10(-6) S cm(-1), is observed for x similar to 0.3. (C) 2003 Elsevier B.V. All rights reserved.