A Hydrogen Vibration Mill (HVM) was designed, constructed and then used to produce Nd-Fe-B powders. The powders were processed to give fully; dense, sintered magnets. When exposed to hydrogen, Nd-Fe-B material undergoes decrepitation, splitting bulk material into highly friable particles, suitable for milling. The supply of hydrogen to the mill was from a LaNi5 store. The mill construction allowed in situ hydrogenation and milling at 0.5-5 bar H-2. This occurred in a stainless steel chamber of 1000 cm(3) capacity with carbon chrome milling balls. The amplitude of vibration was 2-4 mm at 45 Hz. The in situ decrepitation and dry milling facilitated a low oxygen contamination, which is beneficial to sintering and magnetic properties. A milling time of 3 h yielded powders of 7 mu m mean diameter, suitable for producing magnets. The best properties obtained were an energy product of 305 kJm(-3), with coercivity of 704 kAm(-1).