Physical and chemical changes of 90 wt% hydrogen peroxide have been investigated to pressures of 12 GPa by using a diamond-anvil cell, synchrotron x-ray diffraction, and Raman spectroscopy. Hydrogen peroxide freezes at 1.5 GPa and ambient temperature into a tetragonal structure (P4(1)2(1)2, Z = 4, denoted as H2O2-I. This is the same transition that has previously been reported in this material at 253 K. The unit cell parameters at 6.3 GPa are a = 3.759 Angstrom, c = 7.397 Angstrom, and V = 15.74 cm(3)/mol, representing 21% compression from that at ambient pressure. H2O2-I has been found to transform into a high pressure phase H2O2-II at 7.5 GPa, and it decomposes into water and oxygen at the onset of melting, which may be incongruent. In contrast to water, hydrogen peroxide exhibits a relatively simple polymorphism and a positive initial slope of the melting curve at high pressures.