The effect of enriching air with oxygen on the explosion behavior of ammonia and ammonia/methane has been experimentally investigated in a closed 5-l cylindrical vessel. Results have shown that for both ammonia and ammonia/methane, when moving from air to pure oxygen, a transition takes place from a normal deflagration behavior to one that near or at the end is accompanied by a combustion-induced Rapid Phase Transition (cRPT) pressure pulse, an explosion mode characterized by strong oscillations in pressure time histories culminating in over-adiabatic peaks (i.e., values of the peak pressure even higher than 400 bar and, thus, much higher than the thermodynamic value). On the basis of these findings under the experimental conditions applied in our tests, the occurrence of cRPT should be carefully considered in the development of industrial processes based on ammonia reactions in oxygen-enriched atmosphere. For that, the phenomenon should be investigated in equipment of larger volume. (c) 2017 American Institute of Chemical Engineers Process Saf Prog 36: 368-371, 2017