A combined experimental and numerical study was conducted to examine the structure of laminar methane-oxygen diffusion flames in comparison with methane-air flames. Soot measurements made in these flames indicated that the maximum soot yields of methane-air flames are consistently higher than methane-oxygen flames at all pressures. The maximum soot yield of the methane-oxygen flames reaches a peak near 40 atm and then starts decreasing as the pressure further increased. The maximum soot yield of the methane-air flames plateaus at about 40 atm and does not change much with further increases in pressure. Methane-oxygen flames display a distinct two-zone structure which is visible from atmospheric pressure up to 60 atm. The inner zone, similar to hydrocarbon-air diffusion flames, has a yellow/orange colour and is surrounded by an outer blue zone. This outer zone was shown to have a stratified structure with a very steep equivalence ratio gradient. The main reactions in this zone were shown to be the oxidation of hydrogen and carbon monoxide produced within the inner zone. The methane-air diffusion flames had a thin layer of blue outer zone at atmospheric pressure; however, this zone completely disappeared when the pressure was increased above atmospheric. The presence of the two-zone structure in the methane-oxygen flames was attributed to the intensified penetration of oxygen into the core flow. The higher diffusivities, steeper oxygen concentration gradients, and enhanced entrainment increase the transport of oxygen to the flame. As such, there is sufficient oxygen present near the base of the flame to support the diffusion flame in the inner zone of the methane-oxygen flames. The abundance of oxygen near the centerline, even in the lower portion of the flame, also promotes the oxidation of soot. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.