This study investigated chemical decomposition of lignocellulosic components in the course of torrefaction under isothermal conditions for durations up to 5 hours. The goal was a better understanding of the behaviour of biomass, at both short and long residence times, which is important for innovation in the chemical and bioenergy industries. Gaseous and solid-phase decomposition products of cellulose, xylan, and two lignins, were studied following torrefaction at three temperatures (220, 250, and 280 degrees C) for a continuous recording of mass loss and emission of volatiles over 5 hours. Two decomposition stages were revealed for xylan, with a notable release of CO that increased with treatment temperature. 4-O-methyl glucurono-units on the side chains of xylan degraded first, and acetyl groups and macromolecule fragments accounted for the second degradation, starting at 250 degrees C. The primary production of acetic acid occurred at 280 degrees C. For the two lignins, decomposition reactions predominated at lower temperatures, while rearrangement prevailed at 280 degrees C. The emission of phenol was a clear distinction between the two. Cellulose was thermally stable at short times under all treatments, but it decomposed dramatically afterwards, especially at 280 degrees C.