The pressure, temperature reaction phase diagram for ammonium dinitramide (ADN) was determined using a high-temperature-high-pressure diamond anvil cell with optical polarizing light microscopy, Fourier transform infrared spectroscopy, laser-Raman spectroscopy, and energy dispersive X-ray diffraction. The phase diagram was determined between ambient pressure and 10.0 GPa over the temperature range from -75 degrees C to decomposition temperatures, or 120 degrees C. The diagram delineates the melting curve for alpha-ADN, a reversible phase transition in alpha-ADN forming a new high-pressure monoclinic polymorph, beta-ADN, and also identifies the pressure and temperature conditions at which a solid phase rearrangement occurs to form ammonium nitrate (AN) and N2O. Energy dispersive X-ray diffraction and Raman spectra were obtained for both alpha- and beta-ADN as a function of pressure at room temperature.