As a result of interest by the U.S. Department of Energy, the Bureau of Mines conducted studies on comminution of bituminous coal to the 2-um particle size range using the Bureau-developed turbomill. Both "plant-grind" size (80 pct minus 75 um) and microsize (minus 10 um) coal can be produced in a single step with the Bureau's turbomill. In 15 min, minus 2.4-Mm coal milled in water with steel shot was reduced to 65 pct minus 75 um and 26 pct minus 2 um, with an energy requirement of 139 kw.H/mt coal. Plant-grind size coal milled in water, with ottawa sand as the milling medium, was reduced to more than 45 pct minus 2 um in 15 min, at 175 kw.H/mt. Plant-grind coal milled with steel shot was reduced to 57 pct minus 2 um in 15 min, at 138 kw.H/mt. When diesel fuel was substituted for water, the milling of the coal was less effective and less energy efficient. Keeping the pulp dispersed was critical. Without an effective dispersant, the slurry became very viscous and increased the milling time, which resulted in higher energy requirements, low heat dissipation, and poor grinding efficiency.