Considerable amounts of NOx are generated from the thermal utilization of solid wastes. To control NOx emission, we investigated the influence of mixed Fe/Ca additives on the generation of NOx precursors (HCN and NH3) during pyrolysis at 873, 1073, and 1273 K. Protein, proline, and phenylalanine were selected as model compounds to represent the primary nitrogen forms in typical solid wastes, such as biomass and sludge. According to the results, the iron effect is a remarkable property that enables the obvious reduction of NH3. This was most likely because NH3 was consumed by iron compounds to generate FeNx. Iron compounds facilitated thermal cracking of heterocyclic N with the release of HCN center dot Ca(OH)(2) promoted the conversion of HCN to NH3 and fixed HCN to form CaCxNy simultaneously. These intermediates (FeNx and CaCxNy) play important roles in the reduction of NON precursors. A lower emission of NOx precursors was observed with mixed Fe/Ca additives than with either Fe or Ca individually. This is because the generation of Ca2Fe2O5 enhanced the tranformation of intermediates to N-2. The transformation of intermediates to N-2 was enhanced at elevated temperatures, which allows for the lowest emission of precursors (NH3 and HCN) at 1273 K, with a higher ratio of Ca2Fe2O5 in char.