Three processes for the selective non-catalytic removal (SNCR) of nitric oxides from engine exhaust gases are compared. The three processes are similar but each uses a different chemical agent: ammonia, urea, or cyanuric acid. A number of operating conditions have been studied. In particular, results for the removal of nitric oxide are significantly different for the three processes as the oxygen concentration varies. Ammonia, urea, and cyanuric acid were found to be most effective at low, intermediate, and high oxygen concentrations, respectively. The implications of these results for a range of engines and engine applications are discussed.