Efficient lean-burn gasoline engines cannot employ traditional three-way catalysts (TWCs) to control NOX emissions due to excess oxygen. Passive selective catalytic reduction (pSCR) is a process designed to control NOx by generating NH3 over the TWC during fuel-rich operation, storing it on a downstream SCR, and then converting NOx during fuel-lean operation. This work is focused on studying sulfur effects on this promising strategy. Two hydrothermally-aged commercial catalysts were studied for their effectiveness in the presence of SO2: a Pd-based TWC and a TWC with NOX storage. The sulfated samples were evaluated in a flow reactor under pSCR conditions and with targeted reactions. SO2 inhibited the water gas shift and steam reforming reactions, decreasing (but not eliminating) NH3 production; however, initial activity is recoverable after a simple thermal treatment at 650 °C. This work will prove valuable in understanding, mitigating, and even reversing the effects of SO2 inhibition on real pSCR systems.