The solid-state structure of xenon trioxide, XeO3, was reinvestigated by low-temperature single-crystal X-ray diffraction and shown to exhibit polymorphism that is dependent on the crystallization conditions. The previously reported alpha-phase (orthorhombic, P2(1)2(1)2(1)) only forms upon evaporation of aqueous HF solutions of XeO3. In contrast, two new phases, beta-XeO3 (rhombohedral, R3) and gamma-XeO3 (rhombohedral, R3c), have been obtained by slow evaporation of aqueous solutions of XeO3. The extended structures of all three phases result from Xe=O---Xe bridge interactions among Xe03 molecules that arise from the amphoteric donor acceptor nature of XeO3. The Xe atom of the trigonal-pyramidal XeO3 unit has three Xe---O secondary bonding interactions. The orthorhombic alpha-XeO3 displays the greatest degree of variation among the contact distances and has a significantly higher density than the rhombohedral phases. The ambient-temperature Raman spectra of solid alpha-and gamma-XeO3 have also been obtained and assigned for the first time.