On grinding in pure water, zirconium, tantalum iron and stainless-steel powders were extensively comminuted and simultaneously oxidized with hydrogen release, whereas nickel, copper, and silver powders did not react with water and their particle sizes increased. On grinding nickel, copper, and silver in water pressurized with oxygen nickel and copper became extensively comminuted and were oxidized, whereas silver did not react with oxygen and its particle size increased. From these results and other considerations, it is hypothesized that for extensive comminution of ductile metals and alloys to occur on grinding they must react with the grinding media.