The use of pure oxygen in the combustion of fossil-fuels is fuelled by the Urge potential for NOxreduction. This paper describes experiments conducted on a semi-industrial scale test furnace using a single oxy-natural gas burner. The burner was operated in two modes: the oxygen acting as the high velocity fluid, entraining the fuel gas into the flame, and vice versa. A steam-cooled gas-sampling probe was employed allowing gas samples to be taken from inside the furnace. Temperature and species concentration, including NOxwere measured and were corroborated by numerical predictions. This study investigated two main areas, (a) the influence of mixing on the flame characteristics and the formation of nitric oxide in semi-industrial scale burners at high temperature combustion and (b) verification of the model used for high temperature combustion processes. The numerical analysis indicates that reasonable agreement can be obtained for the gas temperature and species concentration, using a 4-step reaction mechanism for the combustion process. The temperature predictions appear to be accurate within ± 50 K and computed NOx, emissions differ at most by 20% from measured NOx, levels. This study shows that NOx, emissions are influenced by the mixing of oxygen and fuel and injection angles. Also fast combustion produced a higher flame temperature and complete combustion but resulted in lower NOx, emissions.