Sulfur breakthrough behaviors during reformate desulfurization were investigated using a novel ZnO-based sorbent with minimized mass transfer resistance. The presence Of CO, CO2, or water affected the breakthrough characteristics of H2S and carbonyl sulfide (COS). CO and CO2 did not significantly affect the reaction between H2S and ZnO, but they reacted with H2S to form COS, which cannot be efficiently removed by ZnO. The mechanisms of COS formation via two different pathways were also investigated. CO reacted with H2S to form COS homogeneously; CO2 reacted with H2S heterogeneously on the sulfide surface. COS formation by CO and CO2 was suppressed by H-2 and water. Water also severely hindered the reaction between ZnO and H2S and significantly decreased H2S breakthrough time. At low water concentrations, sulfur breakthrough was determined by the homogeneous COS formation; at high water concentrations, it was controlled by H2S breakthrough. Capacity loss due to COS formation and adsorption of water was also observed. Novel sorbent and process designs are required to improve the desulfurization performance.