In this paper, we report the chemical stability of a highly proton conducting Sm+Zr-codoped BaCe0.85-xZrxSm0.15O3-delta (BCZS) (0.01 < x < 0.3) in H2O vapor and CO2, and their electrical conductivity in air, N-2 + 3% H2O, H-2 + 3% H2O and N-2 + 3% D2O. All the prepared BCZS was found to be structurally stable against reaction with pure CO2 at 400 degrees C for 24 h, showing a positive role of Zr in providing chemical stability. However, at 600 degrees C for 24 h, BCZS (0 < x < 0.2) showed the formation of BaCO3, while x = 0.3 remained stable at 20% CO2 in N-2. Upon exposure to H2O vapor at 90 degrees C, Ba(OH)(2)center dot xH(2)O formation was observed for all the investigated BCZS whereas at 600 degrees C all were found to be chemically stable. A maximum conductivity of 5 x 10(-3) Scm(-1) is obtained for x = 0.3 member, which is about 40% of its Zr-free parent compound BaCe0.85Sm0.15O3-delta (BCS). Current work shows that despite the drop in conductivity, Zr doping in BCS provides stability under pure CO2, especially considering the goal of decreasing operating temperature of solid oxide fuel cells (SOFCs) below 500 degrees C. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.029301jes] All rights reserved.