High P-T studies of molecular N2O4 using Raman, synchrotron infrared, and X-ray diffraction measurements reveal several new phenomena. Significant changes in the vibrational spectra indicate a phase transition at about 12 GPa. The X-ray diffraction measurements show that the material remains cubic through the transition. We ascribe the transition to a change in the molecular geometry of N2O4 Within the cubic unit cell. Further vibrational spectra of heated samples enabled observation of another phase, the ionic isomer NO+NO3- produced at high pressures and temperatures. These studies provide new information on the phase diagram and stability of N2O4 at high P-T conditions. (C) 2003 Elsevier B.V. All rights reserved.