Biomass pyrolysis for the production of fuels and chemicals is certainly one of the most promising strategies to replace petro-chemical polymers. Generally the biomass is first pyrolysed using temperature up to 700 °C to obtained vapors that are further cracked using catalysts. The catalyst can be poisoned by tars that are formed during the pyrolysis step and that are entrained with the vapor. In order to understand and model the behaviour of the biomass during such a process, a complete structural study of the chars was performed with high-resolution solid-state 13C, EPR, and susceptibility measurements. The origin of biomass does not affect the nature of the solid residues that are formed during the thermal treatment. They all loose their ligno-cellulosic structure and are transformed to polycyclic material with a preponderance of aromatic structures with proton amounts that decrease drastically as the temperature of treatment increases. The presence of unpaired electrons is undoubtedly indicated with EPR spectroscopy. Most of metallic compounds found in the solid residues are easily removed by a mild acidic treatment. It indicates that they are not intercalated inside the polycyclic plans. The occurrence of ferri/ferromagnetic parts has been clearly shown. Their origin is probably exogenous. The study has revealed some non-classical and unexpected features of the NMR spectra that are presented and discussed in relation to the structural properties of the pyrolysed biomass.