In situ high-temperature electron paramagnetic resonance (EPR) has been used to study the pyrolysis (under a helium atmosphere) of cellulose and cellulose/Na2CO3 mixtures, as well as changes in the resulting chars under exposure to O-2 and H2O after quenching to room temperature. It has been found that the addition of Na2CO3 not only dramatically changes the pyrolysis behavior, but also has substantial effects on the subsequent exposure behaviors of the resulting chars to O-2 and H2O. The presence of Na2CO3 substantially narrows the EPR lines of cellulose chars at all charring temperatures used in this study. For low-temperature (<= 350 degrees C) pure-cellulose chars and for cellulose/Na2CO3 mixture chars prepared at all but the highest temperature (550 degrees C) of this study, short-term (<= 120 min) exposure to O-2 has no obvious effect on the spin concentration and the line width; however, decreases in the spin concentration and increases in the line width are observed for high-temperature (>= 400 degrees C) pure-cellulose chars. Effects of H2O introduction into the He/O-2 gas stream are most dramatic for lower-temperature cellulose/Na2CO3 chars and for higher-temperature chars of pure cellulose. The results are discussed in terms of a complex array of possible free-radical reactions.