In recent years, biogas-fed solid oxide fuel cell (SOFC) technology based on Ni-GDC electrolyte-supported cell has attracted interests due to its high sulfur tolerance. However, the degradation and recovery behavior of the cells under exposure to common sulfur compounds remains to be clearly understood especially with electrochemical impedance spectroscopy (EIS). This paper investigates the degradation and recovery behavior of Ni-GDC electrolyte-supported SOFCs under sulfur poisoning via EIS. Two common sulfur compounds in biogas, dimethyl sulfide and H2S, were tested under three gas compositions: dry H2, cold recirculation gas (CO-rich, low H2O), and hot recirculation gas (CO- and H2O-rich). H2S induced greater voltage degradation (35%–45% irreversible) than dimethyl sulfide, which showed complete recovery. Hot recirculation gas led to higher continuous degradation, suggesting that the water-gas shift reaction mitigates long-term poisoning. Lower H2S concentrations caused cumulative irreversible degradation, reducing severity in subsequent higher concentration tests. EIS analysis showed that gas conversion and O2− surface exchange processes were the main contributors to resistance increase, with the latter suffering a 200% rise. This study enhances the understanding of sulfur poisoning effects and EIS behavior in biogas-fed SOFCs, aiding operando operation diagnostics.