Oxygen chemical surface exchange coefficient (k) and bulk diffusion coefficient (D) obtained by electrical conductivity relaxation (ECR) are useful indices to characterize and compare oxygen reduction reaction (ORR) activity of solid oxide fuel cell (SOFC) cathodes. While ECR is an often-used method to characterize a new material’s performance, there has been limited reports on how k and D values change for extended period under fuel cell operating conditions. As performance degradation of the cathode is closely related with changes in surface activity and/or bulk diffusivity, long-term ECR tests and post-mortem microstructural analyses were performed on La-Sr-Co-Fe-O based cathodes to understand the degradation behavior at cathode. In this work, we chose La-Sr-Co-Fe-O based cathodes with different composition (stoichiometric La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) vs. A-site deficient (La_0.6Sr_0.4)_0.95Co_0.2Fe_0.8O_3-δ (LSCF-A)) and with different configurations (unmodified LSCF-A vs. LSCF-A surface modified with Pr_0.6Sr_0.4CoO_3-δ (PSC-LSCF-A)), and monitored variations of k and D values up to 1000 hours. The initial apparent k values followed the trend as PSC-LSCF-A > LSCF-A > LSCF, indicating that A-site deficiency in LSCF improves surface activity. The k values decreased gradually for all the samples over ~1000 hour ECR tests, while the D values were more constant. Oxygen partial pressure dependency of k of the PSC-LSCF-A sample also changed after the long-term tests, indicating a change in oxygen reduction behavior. The samples’ surface chemistry and element ratios before and after the long-term tests showed evidence of Sr-segregation and Co-depletion compared with the original bulk materials. The degradation behavior of k is compared with the degradation behavior from symmetrical cell and single cell performance data. The results imply that controlled ECR measurements can be used to identify surface modification electrocatalysts that can improve surface exchange behavior and mitigate surface degradation.