An understanding of the complex reaction mechanisms involved in the formation of nitrogen oxides from the combustion of fossil fuels provides a basis for the design and application of NOx control strategies. This experimental study is concerned with the formation of HCN and NH3 (as the dominant NOx precursors) from 130 kW turbulent-spray flames operating in standard and externally air-staged modes. Detailed nitrogenous species-concentration measurements from a series of nitrogen-doped gas-oil flames (using pyridine, pyrrole, quinoline, benzonitrile, benzylamine and phenylbenzylamine) supported an NO-formation route where fuel-nitrogen is initially converted to HCN, which subsequently decays to NO via NH3. Although variations were found in the developments in concentration and peak concentration levels of HCN and NH3 with each additive for an equivalent fuel-nitrogen concentration, the differences in the final NO emissions were small. Comparison of the experimental NO concentration profiles from combustion of gas oil with the same fuel containing 0.45% by mass nitrogen (by doping with pyridine) enabled the developments of thermal and fuel-NO to be followed separately for both combustion modes. For an air-staged flame operating at a primary zone (fuel/air) equivalence ratio phi, of 1.21, thermal-NO was reduced by 21% relative to an unstaged flame at the same overall stoichiometry of phi(2) = phi(1) = 0.85. A fuel-NO reduction of 33% was accompanied by significantly increased in-flame production of HCN.