The combustion stability (extinction) limits and nitrogen oxide (NOx) emissions of nonpremixed ammonia (NH3)-hydrogen (H-2)-air flames at normal temperature and pressure are studied to evaluate the potential of partial NH3 substitution for improving the safety of H-2 use and to provide a database for the nonpremixed NH3-substituted H-2-air flames. Considering coflow nonpremixed NH3-H-2-air flames for a wide range of fuel and coflow air injection velocities (V-fuel and V-coflow)and the extent of NH3 substitution, the effects of NH3 substitution on the stability limits and NOx emissions of the NH3-H-2 air flames are experimentally determined, while the nonpremixed NH3-H-2-air flame structure is computationally predicted using a detailed reaction mechanism. Results show significant reduction in the stability limits and unremarkable increase in the NOx emission index for enhanced NH3 substitution, supporting the potential of NH3 as an effective, carbon-free additive in nonpremixed H-2-air flames. With increasing V-coflow the NOx emission index decreases, while with increasing V-fuel it decreases and then increases due to the recirculation of burned gas and the reduced radiant heat losses, respectively. Given V-coflow/V-fuel the flame length increases with enhanced NH3 substitution since more air is needed for reaction stoichiometry. The predicted flame structure shows that NH3 is consumed more upstream than H-2 due to the difference between their diffusivities in air. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.