The objective of this study is to develop a model for the description of torrefaction. The model is conceived in the context of process modelling and therefore it aims at providing coherent mass and energy balances of solid and gaseous products, rather than a physical description of the process. The rationale underlying the proposed model stems from the representation of torrefaction and its products on a C-H-O ternary diagram and its focus is the description of the solid product in terms of yield, composition and heating value. The heating value of the gaseous products is determined by considering water, carbon dioxide and acetic acid as the major volatile products and closing the mass balance. It is possible to extend the number of species considered in the torrefaction gases if experimental data regarding the volatile products are available. The proposed model is simple, of easy implementation and calibration, of fast resolution and its results have been validated against experimental data. This work represents the basis for a future evaluation and optimisation of torrefaction as a pretreatment step in the thermo-chemical conversion of biomass. (C) 2014 Elsevier Ltd. All rights reserved.