Mechanical millings of a Nd-Fe-B powder under reactive hydrogen and inert argon atmospheres were performed. The influence of milling atmosphere and hydrogen pressure on milling products was investigated by X-ray diffraction (XRD) and transmission Mossbauer spectrometry (TMS). Milling under argon leads to a mixing of alpha -Fe, Nd-rich regions and amorphous Fe-B. Upon milling under hydrogen, Nd reacts with hydrogen to form NdH2+x. alpha -Fe and amorphous Fe-B. The dissociation of the elements is more important during HEBM under hydrogen and increases with hydrogen pressure. The differential scanning calorimetry analysis of a powder milled under hydrogen show a two-step desorption of hydrogen from NdH2+x, at temperatures similar to those observed during the DDR process. Powders milled under reactive hydrogen atmosphere were annealed for 0.5 hour, at temperatures between 500 degreesC and 600 degreesC. The Nd2Fe14B phase was found to recombine at 520 degreesC. The presence of secondary phases such as neodymium oxides, alpha -Fe and Nd1.1Fe4Be4 after annealing, shows that the recombination reaction is incomplete, likely due to oxygen contamination during the process.