The nitrogenation and nitrogen disproportionation behavior of NdFe 1 1 Ti and NdFe 1 0 . 7 5 V 1 . 2 5 was investigated by in situ neutron powder diffraction at temperatures between 100 and 800 o C. The nitrogen content and lattice parameters were measured as a function of temperature and time. In both compounds, the nitrogen disproportionation starts before the interstitial modification is complete. The product of the nitrogen disproportionation is a mixture of α-Fe and an amorphous phase, which is stable up to temperatures of 800 o C.