It is well known that conventional solid oxide fuel cells (SOFCs) with oxygen ion conducting electrolyte and nickel (Ni) anode are susceptible to poisoning by trace amount of hydrogen sulfide (H₂S) while not significantly impacted by presence of carbon dioxide (CO₂) in the fuel stream.  In comparison, the impacts of H₂S and CO₂ as fuel contaminants on proton-conducting SOFCs remain unexplored.  Here we report our study aimed at revealing the poisoning behavior caused by H₂S and CO₂ for proton-conducting SOFCs.  Anode-supported proton-conducting SOFCs with BaZe_0.1Ce_0.7Y_0.1Yb_0.1O₃ (BZCYYb) electrolyte and Ni-BZCYYb anode and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathode were subject to ppm-level H₂S or percentage-level CO₂ as fuel contaminants in hydrogen fuel. The response in cell electrochemical behavior and the change in materials microstructure and composition are characterized.  Preliminary results suggest that, contrary to conventional SOFCs that show “sulfur poisoning” and “CO₂ tolerance”, such Ni-BZCYYb anode/BZCYYb electrolyte/LSCF cathode proton-conducting SOFC button cells show good tolerance to H₂S up to ~5 ppmv while severe poisoning by CO₂ at even low percentage level.  Experimental results aimed at understanding such different electrochemical behaviors for proton-conducting SOFCs versus conventional oxygen ion conducting SOFCs will be presented and their implications on the fundamental mechanism for anode hydrogen electrochemical oxidation reaction in proton conducting SOFCs will be discussed.