Granting that the ammonium salts of phosphoric acid are desirable as fertilizers, and that their importance in the fertilizer industry is assured, I undertook this series of experiments in January, 1928 with the idea of obtaining information that would be of benefit to the manufacturer. The results obtained from the series of experiments on the thermal decomposition of both mono and di ammonium phosphate salts give to the manufacturer an accurate comparison of the action of the two salts under the same heating conditions, as well as their individual action. The data obtained from this research are given beginning with Chapter IV.