The adduct, (OsO3F2)(2)center dot 2XeOF(4), was synthesized by dissolution of the infinite chain polymer, (OsO3F2)(infinity), in XeOF4 solvent at room temperature followed by removal of excess XeOF4 under dynamic vacuum at 0 degrees C. Continued pumping at 0 degrees C resulted in removal of associated XeOF4, yielding (OsO3F2)(2), a new low-temperature phase of OsO3F2. Upon standing at 25 degrees C for 11/2 h, (OsO3F2)(2) underwent a phase transition to the known monoclinic phase, (OsO3F2)(infinity). The title compounds, (OsO3F2)(infinity), (OsO3F2)(2), and (OsO3F2)(2)center dot 2XeOF(4) have been characterized by low-temperature (-150 degrees C) Raman spectroscopy. Crystallization of (OsO3F2)(2)center dot 2XeOF(4) from XeOF4 solution at 0 degrees C yielded crystals suitable for X-ray structure determination. The structural unit contains the (OsO3F2)(2) dimer in which the OsO3F3 units are joined by two Os---F---Os bridges having fluorine bridge atoms that are equidistant from the osmium centers (2.117(5) and 2.107(4) angstrom). The dimer coordinates to two XeOF4 molecules through Os-FF center dot center dot center dot Xe bridges in which the Xe center dot center dot center dot F distances (2.757(5) angstrom) are significantly less than the sum of the Xe and F van der Waals radii (3.63 angstrom). The (OsO3F2)(2) dimer has C-i symmetry in which each pseudo-octahedral OsO3F3 unit has a facial arrangement of oxygen ligands with XeOF4 molecules that are only slightly distorted from their gas-phase C-4v symmetry. Quantum-chemical calculations using SVWN and B3LYP methods were employed to calculate the gas-phase geometries, natural bond orbital analyses, and vibrational frequencies of (OsO3F2)(2), (OsO3F2)(2)center dot 2XeOF(4), XeOF4, OsO2F4, and (u-FOsO3F2)(2)OSO3F- to aid in the assignment of the experimental vibrational frequencies of (OsO3F2)(2), (OsO3F2)(2)center dot 2XeOF(4), and (OsO3F2)(infinity). The vibrational modes of the low-temperature polymeric phase, (OsO3F2)(infinity), have been assigned by comparison with the calculated frequencies of (mu-FOsO3F2)(2)OSO3F-, providing more complete and reliable assignments than were previously available.