Lanthanum strontium cobalt iron oxide (LSCF) is a mixed ionic and electronic conducting cathode material used in solid oxide fuel cells (SOFCs) and protonic ceramic fuel cells (PCFCs). However, owing to the lack of understanding of LSCF materials' electrical and transport properties under actual PCFC operating conditions, the general criterion that can determine whether LSCF can be applied to PCFC cathodes has yet to be determined. In this study, the influence of wet atmosphere on the electrical and transport properties of an LSCF cathode was investigated. After changing the gas from dry air to wet air, the electrical conductivity of the LSCF cathode measured via the DC four-probe method increases very slowly, and it reaches a saturated value after about 120 h. The chemical diffusivity (D-chem) measured via the conductivity relaxation technique also decreases to log D-chem = -6.15 +/- 0.01 (cm(2) s(-1)) after the LSCF cathode is exposed to wet atmosphere, which is one order of magnitude lower than that of log D-chem = -4.91 +/- 0.01 measured in dry atmosphere. In the conductivity relaxation experiment, all the relaxation characteristics during a cycle of changes in water vapor activity are found to be caused by oxygen diffusion rather than proton diffusion. Using XPS depth profiling, we demonstrate that the reduction in D-chem and its related sluggish conductivity relaxation in wet atmosphere are caused by the compositional change in the region approximately 100 nm away from the LSCF surface. (C) 2013 Elsevier B.V. All rights reserved.