Low-temperature pyrolysis (torrefaction) is the first step for combustion. The produced vapors and solids undergo combustion with a different mechanism. Therefore, the composition of the vapors and the functional groups of the produced solids are very important for combustion because they directly influence the diffusion and transfer of oxygen. In this paper, low-temperature pyrolysis of pinewood was studied in a fixed-bed reactor and also by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The effects of temperature, flow rate of nitrogen, and atmosphere were considered to understand the distribution and properties of gas, liquid, and solid products. In situ DRIFTS was used to monitor the evolution of functional group on the solid during low-temperature pyrolysis. The results indicated that temperature was the dominant factor that influenced product formation. Gases released were mainly CO and CO2, and their yield increased with increasing temperature; noncondensable volatiles contained acetic acids and furans when the temperature was lower than 250 degrees C, and higher temperatures favored the formation of guaiacols. Characteristic peaks of anhydrides and carboxylic acids showed an irreversible change in the heating and cooling stage, and the change of benzenes can be recovered when cooling to room temperature. This study investigated the evolution of functional groups of biomass during low-temperature pyrolysis, which provided solid feedstock with appropriate chemical structure for combustion.