Although solid oxide fuel cells (SOFC) have demonstrated excellent performance, the durability of SOFCs under real working conditions is still an issue for commercial deployment. In particular cathode exposure to atmospheric air contaminants, such as humidity, can result in long-term performance degradation issues. Therefore, a fundamental understanding of the interaction between water molecules and cathodes is essential to resolve this issue and further enhance cathode durability. In order to study the effects of humidity on the oxygen reduction reaction (ORR), we used in-situ <
sup>
18<
/sup>
O isotope exchange techniques to probe the exchange of water with two of themost common SOFC cathode materials, (La<
sub>
0.8<
/sub>
Sr<
sub>
0.2<
/sub>
)<
sub>
0.95<
/sub>
MnO<
sub>
3�?<
/sub>
(LSM) and La<
sub>
0.6<
/sub>
Sr<
sub>
0.4<
/sub>
Co<
sub>
0.2<
/sub>
Fe<
sub>
0.8<
/sub>
O<
sub>
3-?<
/sub>
(LSCF). In this experiment, heavy water, D<
sub>
2<
/sub>
O (with a mass/charge ratio of m/z = 20), is used to avoid the overlapping of H<
sub>
2<
/sub>
O and the <
sup>
18<
/sup>
O<
sub>
2<
/sub>
cracking fraction, which both provide a peak at m/z = 18. A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of water with the cathode surface as a function of temperature, oxygen partial pressure, and water vapor concentration. The results suggest that water and O<
sub>
2<
/sub>
share the same surface exchange sites, leading to competitive adsorption. Our findings show that water prefers to exchange with LSCF at lower temperatures, around 300?450�C. For LSM, O<
sub>
2<
/sub>
is more favorable than water to be adsorbed on the surface and the presence of O<
sub>
2<
/sub>
limits water exchange. The experimental data are summarized in a Temperature-PO<
sub>
2<
/sub>
diagram to help visualize how the exchange of water on each material depends on the operating conditions.