Large-eddy simulations (LESs) of a hydrogen-enriched 1/3-scale GE7EA gas turbine combustor are conducted. Four different fuel compositions are employed to investigate the role of the CH4/H-2 syngas composition on the resulting flame structure and pressure oscillations occurring inside the combustor. A comparison with the experimental data is conducted to validate the numerical results. First, imaging processing is performed using an Abel-inversion technique for the accumulated OH mass fraction showing good agreement with the experimental images. Then, the calculated velocity fields are successfully compared to the experimental (particle image velocimetry) results. The results show that the flame structure is readily altered when changing the syngas composition; this strongly affects the flow field and therefore the pressure oscillations inside the combustor. When the hydrogen composition is increased, the flame becomes shorter and thicker, and its effect on the outer recirculation zone is minimized. When the flame length approaches the radial length of the combustor under certain conditions, the flame periodically attaches to the rigid wall and the pressure oscillations inside the combustor become amplified. Overall, the LES combined with the multi-step kinetics successfully predicts the variation in the flow fields due to fuel composition changes and reveals the role of the syngas composition in the combustor. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.