This disclosure provides a composition comprising one or more substrate materials (e.g., polymers, rubbers, paper pulps, textiles, or polymer foams) and a flame retardant composition. The flame retardant composition includes at least one flame retardant salt (e.g., an ammonium salt of phosphoric acid such as water soluble ammonium polyphosphate), a nitrogen-containing compound (e.g., urea), and optionally water. This disclosure also provides a process for preparing the flame retardant composition, a process for imparting flame retardancy to a substrate material, and an intumescent process for forming an insulating protective layer on a substrate. This disclosure further provides fire retardant articles and processes for preparing fire retardant articles.