(Ba1-xLax)(2)In2O4+2x(OH)(y) (0 <= x < 0.6; y <= 2(1-x)) compounds were prepared by reacting (Ba1-xLax)(2)In2O5+x square(1-x)(0 <= x <= 0.6) phases with water vapor. This reaction is reversible. A thermal analysis of the hydration process shows that the stability of hydrated phases decreases when x increases. The filling of oxygen vacancies is complete up to x=0.2 and decreases for larger x values to reach 17% only for x=0.5. For x <= 0.3 the incorporation of water is not a single-phase reaction. In wet air at 350 degrees C, a proton contribution to the total conductivity is observed for all compositions but its proportion decreases when x increases. (c) 2007 Elsevier B.V All rights reserved.