A theoretical study of the influence of mixing on the selective non-catalytic reduction (SNCR) process has been performed. The study includes the use of a detailed kinetic reaction mechanism together with a simple approach for mixing based on the "maximum mixedness model" proposed by Zwietering (1959). Two different configurations for that simple mixing approach have been considered and discussed, i.e. the "direct approach" which implies entrainment of the jet flow (containing the SNCR reduction agent) into a bulk flow (containing the products of the main combustion zone); and the "reverse approach" which represents entrainment of the bulk flow into the jet stream. The main features of both approaches applied to the SNCR process are analyzed and discussed. Furthermore, comparison of the results obtained with the present approaches with different experimental pilot scale results is performed.