This paper reports an investigation on low-temperature oxidation prior to the ignition of biomass char from slow pyrolysis with a focus on the impact of inorganic matter. A set of chars was produced from the slow pyrolysis of cornstalk, cellulose, and their pretreated samples (water-leached cornstalk and KCl-doped cellulose) at 500 degrees C. The data for simultaneous thermogravimetric analysis and differential scanning calorimetry and isothermal calorimetry showed that the presence of active inorganic matter, particularly K, in the raw materials enhances the reactivity and favors the heat generation for the low temperature oxidation of the chars. These observations further suggest that water leaching of biomass to remove inorganic species may be an effective strategy to reduce the self-heating tendency of the char. The in situ diffuse reflectance infrared Fourier transform spectroscopy results indicated that active inorganic matter, particularly K, may provide active sites overtaking aliphatic C-H and O-H sites for O-2-char reactions at low temperatures. The presence of active inorganic species, such as K, affects the formation behavior of total C=O groups but appears to have an insignificant influence on the formation of individual C=O groups, which were observed to be more dependent upon the char material.