Differential thermal, thermogravimetric, and derivative thermogravimetric analyses were used to study the effects of two important fire retardant chemicals-ammonium phosphate and ammonium sulfate-on the pyrolysis and combustion of cellulose. To aid in the interpretation of treated cellulose thermograms, the thermal behavior of the fire retardant chemicals was investigated. An increase in the concentration of either flame retardant lowered the threshold temperature and activation energy required to initiate cellulose pyrolysis and combustion, generally decreased maximum weight loss rates, and caused an increase in the production of residue or char. Although these general similarities were found, there were distinct differences in the temperatures at which the rates changed when treated with the same quantity (on a molar fraction basis) of retardant chemical. The difference in which these chemicals alter pyrolysis and combustion is due to a difference in the availability of the inorganic fraction involved in the reaction or to a difference in the reaction mechanism, or both. The study demonstrated that direct comparison of retardant chemicals on the basis of their thermal effects on pyrolysis and combustion at one treatment level could lead to erroneous interpretation and improper classification.