<p>Flame propagation speeds are reported for ammonia/hydrogen/air mixtures with equivalence ratios in the range of 0.5–1.5, preheated gas temperatures ranging from 298 to 673 K and hydrogen volume fractions of 0%, 20%, and 50%. The measurements were conducted using a Bunsen burner and an optical schlieren system. The results show that the flame propagation speed and combustion stabilities of the premixed gases increase with increasing preheating temperature. The combustion stability is significantly improved under the 20% hydrogen volume fraction condition. For the NH₃/H₂ mixtures with a hydrogen volume fraction of 50%, the flame propagation speed at 673 K with a stoichiometric ratio is approximately 4.85 times that at 298 K. The experimental results show that at 673 K, the flame propagation speed of the NH₃/H₂/air mixture increases by 7.8 times when the hydrogen volume fraction increases from 20% to 80%. The numerical results predicted with the Mei, Shrestha, and Stagni mechanisms are compared with the experimental data. The mechanisms proposed by Shrestha and Stagni overestimate the flame propagation speed, especially at high preheating temperatures. The results predicted with the Mei mechanism are consistent with the available data. The concentrations of OH, H, O and NH₂ are increased by the hydrogen addition; thus, the ammonia consumption is accelerated.</p>