The aim of this work was to identify and quantify the chemical and structural modifications occurring during the pyrolysis of Miscanthus × giganteus. Miscanthus was pyrolyzed at slow heating rate (5 K/min) in the temperature range 200–400 ◦C. The biochars were then characterized by combining diffuse reflectance infrared Fourier transform spectrometry (DRIFTs) and Ultraviolet Raman spectrometry (UV Raman). In addition, the quantification of the functional groups remained on the biochars was conducted by temperature programmed desorption-mass spectrometer (TPD-MS) analysis. DRIFT analysis showed that the decrease in the hydroxyl and C H stretching bands and the evolution of the carboxyl vibration peak mainly occur between 300 and 350 ◦C. The evolution of the main markers (H2O, CO, CO2, CH4, and H2) of the functional groups presents in the biochars as analyzed by TPD-MS is consistent with the spectroscopic analysis. It shows two peaks. The first one in the temperature range of 200–380 ◦C was explained by the loss of the main O and H containing groups and to the main primary volatile compounds. The second step (above 380 ◦C) can be interpreted by the rearrangement of the aromatic rings in the polycyclic structure.