The chemical and structural stability of Sm- and In- co-doped proton conductor BaCe0.80-xSm0.20InxO3-delta (x = 0-0.20) in various atmospheres is focused on in this work. All the samples react with CO2 and boiling water to generate barium carbonate and hydroxide. The In doping decreases the CO2-resistance but enhances the liquid water-resistance of the co-doped samples. The powdered samples with x = 0.10-0.20 transform into the monoclinic structure whereas the sintered pellets keep the initial orthorhombic structure after annealed in wet Ar and dry 5% H-2/Ar at 650 degrees C for 6 h. The structure of the powder and pellet samples with x = 0 and 0.05 remains unchanged. This phase transition is related with the dissolution of water vapor and hydrogen into the lattice. It is a very sluggish process for the dense samples and has almost no influence on the electrical conductivity. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.083207jes] All rights reserved.