In this work, the effect of sulphur poisoning of the Ni-YSZ electrode of an SOEC operated in co-electrolysis mode was investigated. Short-term tests with exposure up to 5 ppmv of SO2 were performed at OCV and under polarization (0.25 A cm−2). The two-stages degradation pattern observed consisted of an initial fast voltage increase followed by a slower voltage increase similar to that of an SOFC exposed to H2S. Electrochemical impedance spectroscopy and the analysis of the distribution of relaxation times showed that both the catalytic and electrochemical reactions were affected by SO2. After extended periods in SO2-free reactant, only a partial recovery of the performances was observed even when exposure amounted to only 0.5 ppmv of SO2 independently on the current density. A durability test at a constant polarization of 0.5 A cm−2 showed a voltage ‘runaway’ behavior during successive exposures to 1 ppmv and 2 ppmv of SO2. This behavior originated from a drastic increase of the serial resistance, which almost completely recovered when the SO2 supply was cut. This behavior was not observed during exposure to 0.5 ppmv of SO2, suggesting that, in these test conditions, the voltage ‘runaway’ behavior could be avoided at a sub-ppmv level. Successive exposure-recovery cycles were found to weaken the SOEC tolerance to SO2 and a low frequency pseudo-inductive arc was observed in the impedance response during and after the second exposure to SO2.