Oxygen reduction reaction (ORR) has been intensively investigated due to its relevance to fuel cell applications. It was discovered recently that sulfide (S^2-), a potent poison of Pt, could be a promotor for both ORR activity and stability of Pt. However, the underlying chemistry of working mechanism is still unclear. Understanding the mechanistic effect of non-metallic promotor(s) should help broaden the prospect of developing better ORR catalysts. In this presentation we will discuss the insights we have gained in this respect by in-situ electrochemical infrared spectroscopy and theoretical simulations. Figure 1a and b show the δ(HOO) infrared vibration bands of ORR intermediates HOO_ad and HOOH_ad and v(O-O) vibration of O²⁻_ad under the kinetic potential control regime (at ~0.9 V vs. RHE) on PtS_ad with different sulfur coverage. The IR bands of intermediates HOO_ad and HOOH_ad are larger at lower S_ad coverages at which the enhanced ORR activity was observed. The observation of the O²⁻_ad band at higher S_ad coverage is consistent with the increase in H₂O₂ production during ORR. Interestingly, even a small fraction of S_ad, say a coverage of 0.1, can play a significant role in modifying the ORR reaction pathway by preventing the cleavage of O-O bonding of O₂ molecule. The theoretical simulations in Figure 1c~d confirmed that the turnover frequency (TOF) of H₂O produced at 0.9 V by ORR on PtS_low is much bigger than that on PtS_high. As a conclusion, our finding suggests that the catalyst design by shifting the pathway of ORR from cleavage of O-O bonding on pure Pt to HOO_ad or HOOH_ad pathway would accelerate water production on the fuel cell cathode.