ThMn 12 nitrides are good candidates for high performance permanent magnets. However, one of the remaining challenges is to transfer the good properties of the powder into a useful bulk magnet. Thus, understanding the denitrogenation process of this phase is of key importance. In this study, we investigate the magnetic and structural stability of the (Nd 0.75 , Pr 0.25 ) 1.2 Fe 10.5 Mo 1.5 N x compound (x = 0 and 0.85) as function of temperature by means of neutron powder diffraction. Thermal dependence of the lattice parameters, formation of α-(Fe,Mo), as well as the nitrogen content in the nitrides are investigated by heating the compounds up to 1010 K. The decomposition takes place mainly via the formation of the α-(Fe,Mo) phase, which starts at around 900 K, whereas the nitrogen remains stable in the lattice. Additionally, we show that the magnetic properties of the nitrides [M(4 T) = 90 Am 2 /kg and H c = 0.55 T] are maintained after the thermal treatments up to 900 K. This study demonstrates that the ThMn 12 nitrides with the Mo stabilizing element offer good prospects for a bulk magnet provided an adequate processing route is found.