Oil palm fronds' (OPF) leaves and stems were torrefied under flue gas (produced by flameless burning of wood pellets) and under inert gas (N2) atmospheres using a fixed-bed vertical tubular reactor. The flue gas consisted of N2(73.7-80.2 vol), carbon dioxide (CO2) (12.6-16.5 vol), oxygen (O2) (3.7-4.6 vol), carbon monoxide (CO) (3.7-6.0 vol) and trace amounts of hydrogen (H2) (up to 1.15 vol). Detailed analyses of feed and torrefied products by proximate and ultimate analyses, thermogravimetry and infrared spectroscopy indicated that in the temperature range between 200°C and 300°C, flue gas torrefaction showed enhanced oxygen removal compared to inert gas torrefaction from both feeds, resulting in solid biomass with lower oxygen content and higher energy density. The enhanced oxygen removal under flue gas atmosphere in this temperature range was prominent for hemicellulose/cellulose components and not for lignin. The effect was attributed to O2present in the flue gas, accelerating the decomposition of oxygen-bearing functional groups of hemicellulose/cellulose into water, CO2and CO. Flue gas atmosphere did not affect the volatilisation of biomass in the form of larger organic compounds such as acetic acid, furfural and phenol within the temperature range examined here. © 2023 Lembaga Minyak Sawit Malaysia. All rights reserved.