Ammonium polyphosphate is used as a food additive, E number e545, used as an emulsifier.Ammonium polyphosphates is also used as a flame retardant and as a fertilizer.　As a fire retardant, APP ammonium polyphosphates decompose to form ammonia and phosphoric acid. The phosphoric acid acts as an acid catalyst in the dehydration of carbon-based poly-alcohols, such as cellulose in wood).

Ammonium polyphosphate can be prepared by reacting concentrated phosphoric acid with ammonia. However, iron and aluminum impurities, soluble in concentrated phosphoric acid, form gelatinous precipitates or "sludges" in ammonium polyphosphate at pH between 5 and 7. Other metal impurities such as copper, chromium, magnesium, and zinc form granular precipitates. However, depending on the degree of polymerization, APP ammonium polyphosphate can act as a chelating agent to keep certain metal ions dissolved in solution.

The phosphoric acid reacts with alcohol groups to form heat-unstable phosphate esters. The esters decompose to release carbon dioxide and regenerate the phosphoric acid catalyst. The release of inflammable carbon dioxide helps to dilute the air of oxygen and flammable decomposition products of the material that is burning. The resultant carbonaceous char is less-flammable than before. Use as an intumescent is achieved when combined with starch-based materials.