Mixing ammonia with syngas can be a promising way to overcome the low reactivity of ammonia, allowing it to find usage in IGCC (Integrated Gasification Combined Cycle) systems and gas turbines for power generation. However, fundamental experimental data on laminar burning velocity of NH3/syngas/air are rather scarce, especially at elevated pressures. This information is critical for the development and validation of reaction mechanisms and advances in combustor design. In the present work, measurements of the laminar burning velocities (S-L) of NH3/syngas/air, NH3/CO/air, and NH3/H-2/air premixed flames were performed by the heat flux method at pressures up to 5 atm, equivalence ratios ranging from 0.7 to 1.6, ammonia mole fractions in the fuel mixture from 0.2 to 1.0 in the NH3/syngas/air mixtures and 0.03-1.0 in the NH3/CO/air mixtures. Several recently published ammonia oxidation mechanisms were tested against the present experimental data. The measurements and predictions of SL exhibit discrepancies especially for NH3/H-2/air flames at elevated pressures. The pressure exponent factors, beta, characterizing burning velocity at elevated pressure via empirical power-law correlation S-L/S-L0 =(P/P-0)(beta) are extracted from the measured S-L and compared with the numerical results. The thermal, diffusion, and chemical effects of blending syngas with ammonia on S-L of the mixtures are distinguished, and the dominant role of the adiabatic flame temperature on the variation of the pressure exponent beta is discussed. Kinetic modeling and sensitivity analyses showed that reactions of NHi to N2Hi (1=0-4) species affect the predicted S-L under rich conditions. At elevated pressures, these reactions also affect the NO formation via third-body collision reactions and NHi + NO reactions. Even for rich flames, the ammonia consumption is favored with the addition of syngas which also promotes NO formation by enriching the H and OH radical pools and increasing the flame temperature. The addition of hydrogen or carbon monoxide has equally promoting effect on the ammonia decomposition and NOx formation although their flame speed differs a lot. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.