Solid oxide fuel cells (SOFCs) can generate electricity efficiently and with broad fuel flexibility, but practical applications require them to be more durable and resistant to degradation when subjected to impurities. In order to better understand poisoning mechanisms and guide strategies for developing more durable materials and devices, we have used operando optical methods combined with electrochemical measurements and exhaust gas analysis to investigate the effects of chlorine and sulfur on Ni/YSZ anodes in high temperature (700 °C) SOFCs. The effects of chlorine contamination were found to be more severe for methane than hydrogen operation based on studies of anode-supported Ni/YSZ SOFCs with 100-300 ppm CH₃Cl. Reduced anode activity was indicated by suppressed carbon formation from Raman measurements and fewer oxidation products from infrared emission results. Evidence regarding the degradation mechanism is provided by the differences in recovery for hydrogen and methane. The anode activity is also reduced for operation on methane in the presence of 170 ppm H₂S during both electrochemical and stream removal of carbon. Changes in endothermic methane cracking reactions at the anode are monitored with near infrared thermal imaging while gas and surface species are observed with infrared emission spectroscopy, thereby providing some of the first operando studies of sulfur effects on SOFC anodes.