This study reports the effect of feedstock origin, residence time, and heat treatment temperature on CO2 and O-2 reactivities, nanostructure and carbon chemistry of chars prepared at 1300, 1600, 2400, and 2800 degrees C in a slow pyrolysis reactor. The structure of char was characterized by transmission electron microscopy and Raman spectroscopy. The CO2 and O-2 reactivity of char was investigated by thermogravimetric analysis. Results showed that the ash composition and residence time influence the char reactivity less than the heat treatment temperature. The heat treatment temperature and co-pyrolysis of pinewood char with biooil decreased the CO2 reactivity, approaching that of metallurgical coke. Importantly from a technological standpoint, the reactivities of char from high temperature pyrolysis (2400-2800 degrees C) were similar to those of metallurgical coke, emphasizing the importance of graphitizing temperatures on the char behavior. Moreover, graphitization of chars from wood and herbaceous biomass increased with the increasing heat treatment temperature, leading to formation of graphitizing carbon.