Ammonium sulfate nitrate composite materials useful as fertilizers having desirable levels of nitrate ions, superior stability against detonation, higher density, greater resistance to moisture, and a method for their manufacture. The ammonium sulfate nitrate composites have as essential  constituents ammonium sulfate and the NH4SO4.2(NH4NO3) double salt with less than 5 wt.% in combined total of the more hazardous NH4SO4.3(NH4NO3) double salt and ammonium nitrate.
The composites of the invention are formed by reacting ammonium sulfate with ammonium nitrate in a molar ratio of about 0.9:1 to about 1.1:1 in the presence of a small amount of water in a narrow range of temperatures and then cooling to solidification at a sufficiently rapid rate to prevent macroscopic segregation of the reaction products.