Stuctural transformations of 1,1-diamino-2,2-dinitroethene (FOX-7) were investigated in the temperature range 298-513K by means of DSC, TG, isothermal calorimetry, PXRD, IR spectroscopy, and electron microscopy. The data obtained confirm the existence of the high-temperature -FOX-7 polymorph stable above 480K. The heat effect of the transformation is -4.6Jg(-1) (-680Jmol(-1)). Metastable -phase formed in the reverse process has a perfect crystal structure and is stable towards thermal decomposition. Possible mechanisms of sharp deceleration of thermal decomposition of FOX-7 at the 40% conversion are discussed.