All previous studies on the oxidation of graphene have focused on reaction kinetics. Mass transfer limitation in the O3-led oxidation reaction is important to consider because of the formation of an O2 boundary layer next to graphene. The O3 generator used in this study produces a dilute composition of O3 (8% in O2). Upon oxidation, a molecule of O2 is produced per molecule of O3, which further limits the concentration of O3 near the graphene surface. Computational fluid dynamics (CFD) simulations were performed using COMSOL to understand mass transfer. Figure shows the simulated iso-surface plot of the gas velocity with an inlet flow rate of 2 l min -1 in the tubular reactor where a substrate holding Cu/graphene was placed. The highest velocity was near the center of the reactor. This is because of the small cross-sectional area of the gas delivery system relative to the reactor. The former was essentially a tube with an inner diameter of 2.2 cm.