The paper discusses laboratory premixed flat flame studies showing that, under fuel rich conditions, fuel sulfur can have a radical influence on fuel NO formation mechanisms. Both enhancements and apparent inhibition of fuel NO were observed, depending on the equivalence ratio and the residence time at which the measurement was made. The effect of sulfur on 'prompt' NO was similar. This first order effect was shown to manifest itself in a significant increase in NOx emissions when a fuel containing chemically bound nitrogen is burned with sulfur in an axial (low swirl) turbulent diffusion flame. Under these conditions, in which local hot fuel environments dominate, fuel oils containing both nitrogen and sulfur are likely to give higher NO emissions than those containing little sulfur. Likewise, prompt NO mechanisms can be significant for Thermal NO, and enhancement thereof by SO2 in the fuel can greatly influence resultant Thermal NO emissions.