In-situ high-pressure room temperature synchrotron X-ray diffraction and infrared microspectroscopy were used to examine the structural and vibrational properties and the equation of state of 1,4-dimethyl-5-aminotetrazolium 5-nitrotetrazolate (DMATNT). The X-ray measurements show a smoothly varying pressure-volume relationship to 20 GPa. However, the anisotropic ratios of the unit cell parameters reveal a discontinuity near 3.3 GPa, which can be attributed to an irreversible isostructural phase transition. A significant increase in the Infrared spectral intensity near this pressure coupled with Dayvdov splitting of the NO(2) bending and scissoring modes suggest the transition results in a skewing of the NO(2) groups and increasing asymmetry of the hydrogen bonding sublattice.