The formation of nitrogen oxides from fuel-nitrogen through intermediates was studied by measuring first fuel-O/fuel-N ratios and nitrogen functionality in selected solid fuels. Then the ratios of the yields (fuel-N → HCN)/(fuel-N → NH3) in a nearly inert atmosphere at 800ºC in an entrained flow reactor was measured and finally the ratio (fuel-N → N2O)/(fuel-N → NO) in an oxidizing atmosphere at 800ºC The fuels studied were coal, brown coal, S- and C-type peat, fir bark, birch bark and pine bark, all milled to a particle size <63μm. The ratios of O/N in the fuel, measured by elemental analysis, ranged from 7 to 150. Nitrogen functionality (mass percent of the total nitrogen content) was determined by XPS. the (fuel-N → NCN)/(fuel-N → NH3) conversion ratio in the absence of O2, and also the (fuel-N → N2O)/(fuel-N → NO) conversion ratio with O2 present, decreased with increasing ratio of fuel-O/fuel-N, but neither ratio decreased regularly with the increasing ratio of pyrrolic to pyridinic nitrogen in the fuel. Thus, fuel-oxygen plays a more important role than nitrogen functionality in the chemistry of nitrogen oxide formation. The strong effect of (fuel-O/fuel-N) ratio on the (fuel-N → HCN)/(fuel-N → NH3) ratio may be due to the reaction between OH radicals and HCN to form NH3 near the fuel particle. The importance of this reaction is considered. Charring the fuel sample before combustion led to a sharp drop in the conversion of fuel-N to N2O compared with the virgin fuels. Thus, heterogeneous combustion reactions produced much less N2O than homogeneous combustion reactions.