Wheat and barley straws, corn stover and switchgrass at two moisture. contents were ground using a hammer mill with three different screen sizes (3.2, 1.6 and 0.8 mm). Energy required for grinding these materials was measured. Among the four materials, switchgrass had the highest specific energy consumption (27.6 kW h t(-1)), and corn stover had the least specific energy consumption (11.0 kW h t(-1)) at 3.2 mm screen size. Physical properties of grinds such as moisture content, geometric mean diameter of grind particles, particle size distribution, and bulk and particle densities were determined. Second- or third-order polynomial models were developed relating bulk and particle densities of grinds to geometric mean diameter within the range of 0.18-1.43 mm. Switchgrass had the highest calorific value and the lowest ash content among the biomass species tested. (C) 2004 Elsevier Ltd. All rights reserved.