A comparison between the properties of chars produced by pyrolysis of rice husk and eucalyptus at different temperatures and heating rates has been performed. Low heating rate (LHR) devolatilization experiments were conducted in a fixed bed reactor at temperatures ranging from 600 to 900 °C, while a fluidized bed reactor was used for preparing chars at high heating rate (HHR) and temperatures of 800 and 900 °C. The morphological changes in carbonaceous solids produced in the different thermal treatments were observed by scanning electron microscopy (SEM). X-ray diffraction (XRD) and Raman spectra were obtained to evaluate the degree of char structural order. The chars were characterized by their ultimate analysis, oxygen functional group content, and CO2 adsorption at 0 °C using the Dubinin–Radushkevich method. The results obtained from the different techniques were contrasted to give an overview of the chemical and physical properties of the biomass char samples studied. The influence of the parent material and char properties on char reactivity toward O2 and NO reduction was further investigated. It was found that, though rice husk chars have a greater reactivity toward oxygen, the NO reduction ability was significantly higher for the eucalyptus chars.