Abstract: Various combustible materials such as paper, wood, leather, cloth and textiles of animal or vegetable origin and, especially, cellulosic materials suitable for insulation may be rendered fire retardant by treating the same with an aqueous solution comprising about 10 to 60 per cent of a blend of ammonium nitrate liquor and urea liquor, this blend can be within a range of 44.5% ammonium nitrate, 35.4% urea, and 20.3% water to 39.5% ammonium nitrate, 30.5% urea, and 30% water; about 10 to 50 per cent of an ammoniated poly phosphate having approximately 10 per cent nitrogen and approximately 34 per cent phosphoric acid; about 0 to 25 per cent molasses, starch or sugar or mixtures thereof; and up to about 3 per cent of a strong base such as sodium hydroxide.