Switchgrass was pelleted to evaluate the effect of densification on acidic and alkaline pretreatment efficacy. Bulk density and durability of pellets were 724 kg/m(3) and 95%, respectively. Ground switchgrass (D-90 = 21.7 mm) was further ground to a fine power (D-90 = 0.5 mm) in the pellet mill prior to densificadon. This grinding increased enzymatic hydrolyzate glucose yields of non-pretreated materials by 210%. Pelleting had no adverse impact on dilute acid pretreatment efficacy. Grinding and pelleting increased hydrolyzate glucose yields of switchgrass pretreated by soaking in aqueous ammonia (SAA) by 37%. Xylose yields from SAA-pretreated switchgrass pellets were 42% higher than those from the original biomass. Increases in sugar yields from SAA-pretreated pelleted biomass are attributed to grinding and heating of biomass during the pelleting process. Potential transportation, storage, and handling benefits of biomass pelleting may be achieved without negatively affecting the downstream processing steps of pretreatment or enzymatic hydrolysis. (C) 2012 Elsevier Ltd. All rights reserved.