This paper summarises the results from a series of experimental studies aimed at establishing the effects of fuel-sulphur on the formation and destruction pathways of thermal and fuel-NO in turbulent-diffusion flames. The experiments were performed in a 150 kW(t) pilot-scale test furnace under practical operating conditions and temperatures, so as to be applicable to real systems. Most of the tests were conducted with gas oil, and fuel-N and fuel-S were simulated by quinoline and tetrahydrothiophene (thiophane) respectively. Although this does not exactly replicate the complex compounds of the types found in heavier liquid and solid fuels, it allows for controlled tests to be undertaken while retaining the characteristics of a turbulent-diffusion flame. The NO emissions in the exhaust gas were affected by the addition of sulphur to the fuel, to an extent dependent on the fuel sulphur:nitrogen ratio and mixing conditions in the near-burner region. The magnitude of the effects was greatest for fuel-rich conditions-for example when the flame was staged. Thermal-NO emissions were reduced with sulphur addition for fuel-lean operation of the primary stage, but were unaffected when the furnace was operated fuel-rich. These effects are substantiated by in-flame concentration measurements of NO and the important NO, precursors, ammonia and cyanide, which are presented together with data from comparative studies using SO, as the sulphur additive. The in-flame measurements of ammonia and cyanide showed that where increases in fuel-NO were observed due to the effects of sulphur, there were corresponding reductions in ammonia and cyanide. Kinetic studies on a representative flame system made by means of a PSR approach through CHEMKIN and KINALC showed qualitative agreement with the experimental trends, and demonstrate the main pathways for sulphur-nitrogen interactions. A practical implication of these results for NO, modellers is to raise the need for greater awareness of the role of chemistry, particularly in low-NO, burner applications, since the sulphur appears to be reducing the efficiency of the NO reduction processes in the locally fuel-rich regions of the flame.