The associated mixture model has been applied to describe equilibria in the Fe-rich corner of two ternary Fe-X-N system (with X = Al or B). It has been found that at 1873 K more than 90% of nitrogen is complexed into AlN molecules (associates) in the Fe-Al-N system, when the molar ratio of aluminum to nitrogen is higher than 2.5. On the other hand, in the Fe-B-N system 37% of boron is complexed into BN molecules (associates) at 1823 K, when the nitrogen content of the melt is above 80 ppm, and the boron content of the melt is below 60 ppm. The phase boundaries between liquid iron and N-2 gas on the one hand, and between liquid iron and solid AlN and BN phases on the other hand have been determined. The solubility of N-2 gas increases with increasing concentration of aluminum and boron in the liquid iron. The concentration dependence of the solubility of AlN and BN considerably differs from that predicted by the concept of the,solubility product", due to the strong complex (AlN and BN) formation in the systems.