Torrefaction is a pretreatment method for upgrading biomass as solid fuels. To provide flexible operations for effectively upgrading biomass at lower costs, the aim of this study was to investigate the properties of oil palm fiber and eucalyptus pretreated in nitrogen and air atmospheres at temperatures of 250-350 degrees C for 1 h. Based on energy and solid yield and introducing an energy-mass co-benefit index (EMCI), oil palm fiber pretreatment under nitrogen at 300 degrees C provided the solid fuel with higher energy density and less volume compared to other temperatures. Pretreatment of oil palm fiber in air resulted in the fuel with low solid and energy yields and is therefore not recommended. For eucalyptus, nitrogen and air can be employed to upgrade the biomass, and the suggested temperatures are 325 and 275 degrees C, respectively. (C) 2012 Elsevier Ltd. All rights reserved.