The thermal evolution at a heating rate of 3 degrees C min(-1) of phosphorodiamidic acid, HPO2(NH2)(2), was studied up to 600 degrees C. Thermogravimetric analysis revealed three stages at 120, 320 and 600 degrees C. Nuclear magnetic resonance and Fourier transform-infrared analysis have been used to characterize the thermal products. At 120 degrees C, phosphorodiamidic acid condenses without any weight loss into an ammonium salt of P,P’-diamidoimidodiphosphoric acid. It is transformed at 320 degrees C into a more condensed product containing 17.7 wt% nitrogen and showing P-NH-P and P-O-P linkages. At 600 degrees C, the product still contains 10 wt% nitrogen. Phosphorus nuclear magnetic resonance shows that it is composed of nitrogen-containing Q(3) groups and ultraphosphate Q(3) groups. It is concluded that nitrogen cannot be held in the phosphate network if it contains hydroxyl groups, and that incorporation of nitrogen requires both reducing and nitriding conditions.