An experimental and theoretical investigation of mixing in the SNCR process was performed. The experiments were carried out in a bench scale reactor using the flue gas from a natural gas burner as the main gas and injection of a jet of NH3 mixed with carrier gas in crossflow. The results show a dependency on the carrier gas flow at temperatures above the optimum temperature for NO reduction. No dependency on the variation of the O-2 concentration in the carrier gas from 0 to 21 vol% was observed. It was found that an increasing momentum ratio of the jet to the main gas improves the NO reduction up to a limiting value of the momentum ratio of approximately 20. Above this value no further improvement was observed. Chemical kinetic modelling of the initiating reactions involving NH3 showed that the reaction with OH radicals is the primary initiating reaction. It was also shown that process performance is influenced by the O-2 concentration in the flue gas. The experimental results were used to verify the droplet diffusion model proposed by Ostberg and Dam-Johansen (1995, Chem. Engng Sci. 50, 2061-2067), using an empirical kinetic model valid for 4 vol% O-2 in the reacting gas.