There is a need to reduce NOx emissions, which can only be achieved through a detailed understanding of the mechanisms for their formation and reduction. In this work the release of the NOx precursors, NH3 and HCN, for different fuels is experimentally analysed and modelled in typical fixed-bed combustion conditions. It is shown that NH3 and HCN are released during the main devolatilization phase and the NH3/HCN ratio increases for fuels with a higher nitrogen content. A simplified two-steps model for their release is presented. The model can predict with a reasonable accuracy the release for fuels with a low nitrogen content, however deviations are present for fuels with a high nitrogen content, which probably arise due to a reduction of NH3 and HCN taking place already in the bed.