The coating effect of a graphene layer on the reconstructed Pt(100) surface toward reactive gases is investigated by a combination of scanning tunneling microscopy experiments and density functional theory calculations. We observe that while the graphene coating is effective toward individual exposures of O2 and H2S, a sequential dose of O2 and H2S causes a breakdown of the coating effect. From density functional theory calculations we explain these observations at the atomic level. We show that it is favorable for oxygen atoms to disrupt the Pt–C bonds at the graphene edge to insert between the edge and the surface in a bridging position. This closed-edge configuration explains why oxygen alone is not found to intercalate under the graphene at room temperature. With the subsequent dosing of H2S, we propose that the oxygen atoms are released from the platinum surface causing the breakdown of the coating effect