Ponderosa pine needle and aspen excelsior fuel beds, chosen because they exhibit different chemical fuel characteristics, were treated with various amounts of ammonium sulfate and ammonium phosphate and burned in a wind tunnel under controlled environmental conditions. The rate of fire spread, rate of weight loss (equivalent energy release rate), and the residue were measured to quantify the retardants' effects on flammability. An increase in the amount of either fire retardant chemical caused a reduction in rate of fire spread and rate of weight loss while increasing the amount of residue. However, at all treatment levels, ammonium phosphate was more effective than ammonium sulfate. These two chemicals exhibit their primary difference in their effect on glowing combustion. The rate of weight loss and especially the residue are parameters that quantify glowing combustion. Ammonium phosphate is much more effective in decreasing the rate of weight loss and increasing the residue than is ammonium sulfate, which has little effect on the residue. The superior effectiveness of ammonium phosphate is probably due to the difference in thermal behavior of the two fire retardants; total decomposition of ammonium sulfate occurs prior to 450 C., about 250 C. sooner than ammonium phosphate. Temperatures at the surface of the fuel bed probably are within the range of these total decomposition temperatures. Although ammonium sulfate is effective in retarding flaming combustion, the majority of the chemical may decompose prior to the glowing phase and thus not be available to alter reactions taking place during glowing combustion. The study provides general relationships which can serve as standards for comparing the effects of other basic chemicals or forest fire retardant additives.