Torrefaction of oil palm empty fruit bunches (EFB), waste products from the palm oil industry, was carried out in a fixed-bed tubular reactor in the presence of oxygen at concentrations ranging from 0% to 15%. The effects of oxygen concentration (0%, 3%, 9% and 15%), temperature (493, 523 and 573 K) and biomass size (0.375, 1.5, 3 and 6 mm) on the torrefaction mass and energy yields were investigated. The mass yield decreased with increasing temperature and oxygen concentration, but was unaffected by biomass size. The energy yield decreased with increasing oxygen concentrations, but was still between 85% and 95%. The energy yield showed concave profiles with increasing temperature. Mechanistically, it was found that the 'oxidative torrefaction' process occurred in two successive steps or via two parallel reactions, where; one reaction is ordinary torrefaction, and the other is oxidation. Analysis of the reaction kinetics based on the suggested mechanisms produced an activation energy of torrefaction, that was consistent with previous studies. (C) 2011 Elsevier Ltd. All rights reserved.