A new approach is proposed to retrace the combined effects of temperature and duration within the thickness of heat-treated Fagus sylvatica wood. Torrefaction is a mild pretreatment of biomass carried out at 200e300 _C to improve its properties for pulverized systems such as gasification. The properties of wood treated at high temperature are closely related to chemical modifications induced by temperature levels and treatment duration. This study involved the spectral analysis of solid wood in the near infrared range with the aim of developing a predictive model for process assessment. Samples of beech wood were used for calibration under high temperature conditions of 220, 250 and 280 _C for 1 and 8 h. For prediction, a 50-mm thick solid piece of wood was treated at 250 _C for 3 h. It was demonstrated that it is possible not only to distinguish between wood samples that have undergone different heat treatments, but also to retrace the thermal history of a piece of wood. Statistical processing showed the compensatory effects of temperature and duration, along with the existence of an exothermal reaction in the solid piece of wood. It should thus be possible to ensure cheaper and faster quality control in continuous torrefaction processes.