In this study, experimental and numerical investigations were conducted to study the attachment and oxidation process of laminar CO/H-2 diffusion flames burning in coflow O-2/H2O at 1 atm with an inlet temperature of 400 K for both the fuel and oxidizer streams. The effects of fuel composition were investigated by considering a wide range of CO/H-2 mole ratio from 95% CO-5%H-2 to 5%CO-95%H-2 and also pure H-2. The oxidizer has a fixed composition of 75%H2O-25%O-2. The measured flame heights determined by OH*-chemiluminescence images were used to validate the flame model adopted in this work. Through numerical simulations using a two-dimensional flame code with the preheating effect, detailed reaction mechanism, and detailed thermal and transport properties, the details of flame attachment and flame structure were obtained and analysed. Although both CO and H-2 diffuse over the burner rim and move upstream into the oxidizer stream, the attachment point of a H-2-rich syngas flame is further upstream below the burner exit than that of a CO-rich flame. This is attributed to the high reactivity of H-2 through reaction OH + H-2 = H + H2O and the high diffusivity of H-2. Reaction pathways for syngas burning in the oxidizer of O-2/H2O based on a detailed kinetics analysis were revealed, not only inside the fuel tube and above the fuel exit, but also near the flame sheet and in the flame attachment zone. Significant consumption of H2O was observed in the flame core due to the reverse reaction of OH + H-2 = H + H2O which shifts to proceed forward outside the flame in the radial direction also at higher streamwise locations if H-2 in the fuel flow is rich, oxidizing unburned H-2 to H2O. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.