In this study, an experimental investigation of lean premixed syngas/air flames with H-2/CO ratio of 1.0 and equivalence ratio of 0.5 has been conducted in a high-pressure burner facility to investigate the effects of pressure and the presence of hydrocarbons on NOx speciation. Detailed NOx speciation measurements in the post-flame region were conducted for various pressures up to 1.5 MPa (15 bar) using Fourier transform infrared (FTIR) spectroscopy. When the pressure is increased, NO concentration decreases while NO2 increases due to pressure dependence of NO to NO2 conversion. For a given pressure, the presence of hydrocarbons in syngas leads to an increase in NOx concentrations possibly due to prompt NO formation. Comparison of NO concentrations in presence of CH4 at different pressures shows that the effect of CH4 due to prompt NO formation is more dominant than the effect of pressure on NO. (C) 2018 American Institute of Chemical Engineers