The role of CuO in reducing the volume change and altering the transition temperature of structural phase transitions in ammonium nitrate is studied by electron paramagnetic resonance (EPR). At room temperature, two distinct EPR spectra, indicative of two chemical forms of copper were observed in ammonium nitrate heated from the melt containing 5% by weight of CuO and through which NH3gas was bubbled. One spectrum having an orthorhombicgtensor with principle componentsg1= 2.185,g2= 2.109, andg3= 2.055 is assigned to the Cu(NH3)2+4complex. The other spectrum, also having orthorhombic symmetry, withg1= 2.315, andg2= 2.264, is tentatively assigned to a Cu2+ion either interstitial or substitutional in the lattice. Thegvalues of this spectrum were quite sensitive to temperature and change substantially during the structural phase transition at 60 °C. Heating ammonium nitrate to 160 °C for an hour changed the tetramine copper complex into a new spectrum which is identified as the diamine complex of copper. At room temperature this spectrum slowly returned to the tetramine complex. The role of these forms of copper in reducing the volume change associated with the above room‐temperature phase transition is discussed.