The pyrolysis of guanylurea dinitramide (GUDN), often referred to as FOX-12, was assessed using differential scanning calorimetry together with in situ microscopy and Raman spectroscopy, in order to determine the pyrolysis mechanism for this compound. The experimental results showed that the thermal behavior of GUDN depended on the heating rate, and by the presence of a condensed phase prior to a sharp exotherm at the highest heating rate. During the pyrolysis of GUDN, guanidium nitrate (GN) and cyanuric acid were generated by solid-phase reactions that were associated with this exotherm, and molten GN formation promoted the exothermic reaction.