A program was conducted to determine whether sufficient quantities of the flame retardant, ammonium phosphate, could be incorporated in a water-based foam solution to prevent or delay reignition (burnback) without significant degradation of foam properties. Up to 20% of monoammonium and diammonium phosphate was added to a high- expansion foam solution. This mixture was applied to burning wood and coal samples. The effect of the mixture on burnback was laboratory tested by placing the wood or coal sample in front of a radiant panel. It was found that diammonium phosphate levels of 20 wt-pct prevented reignition without degrading foam properties. A field test was then conducted using a 20% diammonium phosphate mixture and a control with no phosphate on a wood crib fire. The experimental foam did extinguish the crib fire faster than the control as evidenced by the smoke from the crib; however, neither crib reignited.