The effects of fuel sulphur on thermal and feel-NO formation and destruction processes in liquid fuel, spray flames were studied using quinoline and tetrahydrothiophene to simulate fuel-N and fuel-S respectively. The exist gas NO emissions were found to be affected by the addition of sulphur to the fuel to an extent which was dependent on the fuel sulphur to nitrogen ratio, burner operating conditions and fuel type. The increase in fuel-NO emission, at a particular operating condition, due to sulphur addition, was in most cases linearly dependent on the sulphur concentration in the fuel. When the burner was operated in low NOx mode by staging the combustion air, the interactions were found to be dependent on the primary-zone fuel/air equivalence ratios, with greater enhancement observed for fuel-rich conditions. A practical consequence of these results is that the possible reductions in the NO emission by staging are dependent among other factors on the sulphur content of the fuel since the sulphur appears to be reducing the efficiency of the NO reduction processes in the locally fuel-rich regions of the air-staged flame. Thermal-NO emissions were reduced with sulphur addition for fuel-lean operation of the primary stage but were relatively unaffected when operating fuel-rich. Comparative studies using SO2 as the sulphur additive produced similar results, but the magnitude of the changes in the NO emission was less than that found using tetrahydrothiophene. The effect of sulphur on the NO emissions from combustion of other liquid fuels (containing natural, rather than added fuel nitrogen) was also studied using tetrahydrothiophene, tert-butyldisulphide and SO2 as the sulphur dopants.