Solid oxide fuel cells (SOFC) have been demonstrated with high efficiency in energy conversion through electrochemical reactions. However, the durability of SOFC cathodes under real operating conditions is still an important issue for widespread implementation. Chromium-containing interconnect alloys have been reported to cause severe degradations and restrain cathode performance. Also, the presence of both chromium and water vapor have been shown to accelerate the degree of chromium poisoning. Therefore, fundamental understanding of chromium effects, as well as cross-contamination effects, on the oxygen reduction reaction (ORR) and the cathode degradation mechanism are important to preventing chromium poisoning. Here we utilize in-situ gas phase isotopic oxygen exchange to explore effect of chromium on the ORR mechanisms. Gas phase isotopic oxygen exchange is a powerful technique to investigate surface reaction kinetics and catalytic activity from a molecular point of view. To investigate Cr effects on the surface kinetics of cathodes, La_0.6Sr_0.4Co_0.2Fe_0.8O_3-x (LSCF) powders had been aged in contact with Cr-containing alloy (Crofer APU 22) with the presence of water. Aged La_0.6Sr_0.4Co_0.2Fe_0.8O_3-x (LSCF) powders were examined under different atmospheres and the effects of chromium on LSCF as a function of temperature with or without the presence of water vapor were explored. The use of gas phase isotopic oxygen offers separate examination of Cr effects on the surface dissociation step and the overall surface exchange on LSCF. The changes in active surface sites of aged LSCF could also be quantified. Cross-contamination and the lifetime of intermediate adsorbed species were further determined to elucidate multiple gas-gas and gas-solid reactions on cathodes. Results of surface kinetics show correlations with surface configurations of LSCF.  A possible degradation mechanism has been proposed to illuminate chromium poisoning effects on the ORR.