Torrefaction of oil palm kernel shell (PKS), one of the biomass residues from the palm oil industry, was carried out in a fixed-bed tubular reactor in the presence of oxygen and carbon dioxide at concentrations ranging from 0 to 15 vol. (nitrogen balance). The effects of oxygen and carbon dioxide concentrations (0, 3, 9, 12, and 15 vol.), temperature (493, 523, and 573 K) and biomass size (0.375 mm and unground) on the solid phase conversion, the energy yield and properties of torrefied biomass were investigated. The solid phase conversion increased with increasing temperature and oxygen or carbon dioxide concentration, but was not significantly affected by biomass size. The energy yield decreased with increasing oxygen or carbon dioxide concentration, but was still more than 70. The extent of torrefaction was in the order of oxygen > carbon dioxide > nitrogen. The 'oxidative torrefaction' rate was extracted from the overall reaction rate. The increase in the oxidative torrefaction rate caused by oxygen was higher than that caused by carbon dioxide. Scanning electron microscope observations of the morphology of the PKS showed distinctive differences for torrefaction in nitrogen, oxygen, and carbon dioxide. © 2014 Elsevier Ltd.