Biomass is increasingly considered as an important resource for alternative fuels with significant environmental advantages. Recent studies in the literature highlight the importance of the structural modifications which occur after biomass devolatilization is performed under different conditions and their effect on char reactivity. In this context, this work deals with the characterization of chars obtained by the pyrolysis of eucalyptus at different temperatures and heating rates and the influence of their morphological changes on overall reactivity towards oxygen. Pyrolysis experiments at low heating rate (LHR) were carried out in a fixed bed reactor, in the range of 600–900 °C, while a fluidized bed reactor was employed for preparing chars at high heating rate (HHR), at 800 and 900 °C. Results relating to the elemental chemical composition of the chars are reported. In addition, pore volume and surface area of char particles were determined by CO2 adsorption at 0 °C using the Dubinin–Radushkevich method. Chars were also observed by scanning electron microscopy (SEM) in order to visualize the differences in macro-morphology. LHR and HHR chars obtained at 900 °C were selected in order to investigate their oxidation behaviour and these results are discussed in terms of ultimate, surface area and SEM analyses.