Common contaminants in solid oxide fuel cell (SOFC) fuels, including chlorine and sulfur, are believed to induce anode degradation through one of several, temperature-dependent mechanisms. At relatively low operating temperatures, mechanisms propose that contaminants adsorb to the Ni electrocatalyst, blocking active sites and reducing conversion efficiency. This effect is largely reversible and anodes recover performance when the contaminant is removed from the fuel feed. At higher temperatures, contaminants react with Ni to form non-conducting or volatile materials, thus impeding electrochemical oxidation and destroying anode microstructure. Using both electrochemical measurements and operando vibrational Raman spectroscopy in SOFCs operating at 700°C, we have discovered that small amounts of excess molecular hydrogen added to both methane contaminated with chlorine and a biogas simulant contaminated with chlorine masks, but does not prevent, Cl-induced anode degradation. Once the hydrogen is removed from the incident fuel feed, anodes fail abruptly and irreversibly, implying that Cl-induced degradation proceeded while the hydrogen was present despite impedance and voltammetry data showing no apparent signs of performance loss. Effects are more pronounced and occur more rapidly in SOFCs operating with the biogas simulant (a 50-50 mixture (by mole fraction) of CH₄ and CO₂) than for methane. These results are discussed in terms of a new high-temperature degradation mechanism that considers site specific catalytic activity on Ni surfaces and steric requirements for –CH and H₂ bond activation.